S.  Department  of  Boriculture, 


WEATHER  BUREAU. 


INSTRUCTIONS 


FOR 


VOLUNTARY  OBSERVERS 


PREPARED  UNDER  DIRECTION  OF  THE  CHIEF  OF  THE 
WEATHER  BUREAU. 


T.    RUSSELL, 


PBOFESSOB  OF 


PUBLISHED  BY  AUTHORITY  OF  THE  SECRETARY  OF  AGRWULTU&S. 


WASHINGTON,  D.  C.: 

GOVERNMENT    PRINTING    OFFICE. 

1892. 


ZIU  S.  Department  ot  Bgriculture, 

WEATHER  BUREAU. 


INSTRUCTIONS 


FOR 


VOLUNTARY  OBSERVERS 


PREPARED  UNDER  DIRECTION  OF  THE  CHIEF  OF  THE 
M  ^ .       WEATHER  BUREAU. 


BY    T.    RUSSELL, 

PBOFESSOB  or  METEOROLOGY. 


PUBLISHED  BY  AUTHORITY  OF  THE  SECRETARY  OF  AGRICULTURE. 


WASHINGTON,  D.  C.: 

GOVERNMENT    PRINTING    OFFICE 

1892. 


GIFT 


•.  .•:*.;  :  : 


TABLE  OF  CONTENTS. 


Page. 

Introduction 9 

Voluntary  observer's  form  of  application 11 

Thermometers i 12 

Minimum  thermometers 15 

Maximum  thermometers 18 

Instructions  for  setting  up  maximum  thermometers 20 

Instructions  for  setting  up  minimum  thermometers 20 

Thermometer  shelter 22 

Window  or  wall  shelter J -• 23 

Daily  mean  temperature 24 

Solar  radiation  thermometer 25 

Thermoscope 26 

Water  vapor  in  air 27 

Psychrometer 28 

Sling  psychrometer 29 

Hair  hygrometer 30 

Barometer , 30 

Vernier 33 

Correction  of  barometric  observations 38 

Aneroid  barometer 41 

Hypsometer 42 

Rain  gauge 43 

Instructions  for  use  of  rain  gauge 44 

Rain  gauge  support 46 

Percolation  gauge 49 

Wind  vane 50 

Anemometer - 51 

Clouds 55 

Evaporation ..." 57 

River  gauge    58 

State  of  weather 60 

Frost - 60 

Coronas 60 

Thunderstorms — 61 

Tornadoes  and  Sand  spouts 61 

Instructions  for  observing  Auroras 61 

Earthquakes 64 

General  phenomena  of  Climate 66 

Special  instructions  to  voluntary  observers 67 

Instructions  in  the  use  of  the  tables 69 

Table     I.  Temperature  of  the  dew  point 71 

II.  Relative  humidity,  per  cent 81 

III.  Correction  of  vapor  pressure 91 

IV.  Vapor  pressure  in  inches 92 

V.  Grains  of  water-vapor  in  a  cubic  foot  of  air 93 

VI.  Reduction  of  barometer  readings  to  32° 94 

VII.  Reduction  of  observations  of  barometer  to  sea  level 96 


795§8 


LIST   OF  ILLUSTRATIONS. 


Page. 

Figure    1.  Maximum  and  minimum  thermometers 19 

2.  Design  for  thermometer  shelter 22 

3.  Solar  radiation  thermometer 25 

4.  Alcohol  radiation  thermometer 25 

5.  Mounting  of  barometer 31 

6.  Vernier  35 

7.  Vernier   35 

8.  Vernier   35 

9.  Barometer  cistern  and  scale 37 

10.  Barometer  cistern  section 37 

11.  Aneroid  barometer 41 

12.  Eain  gauge,  front  view 45 

13.  Kain  gauge,  vertical  section 45 

14.  Rain  gauge  receiver 45 

15.  Rain  gauge  receiver,  horizontal  section 45 

16.  Rain  gauge  and  support 46 

17.  Wind  vane 50 

18.  Anemometer 51 

19.  Anemometer  dial 54 

20.  Piche  evaporometer 58 

21.  River  gauge 59 

7 


INSTRUCTIONS  TO  VOLUNTARY  OBSERVERS  OF  THE  WEATHER 

BUREAU. 


INTRODUCTION. 

To  render  the  meteorological  observations  made  over  the  United 
States  of  the  greatest  value,  and  for  ease  and  facility  in  their  use  in 
the  investigation  of  questions  relating  to  the  weather,  it  is  important 
that  they  be  made  on  a  uniform  plan  and  the  records  kept  on  some 
uniform  system. 

With  a  view  to  accomplishing  this,  a  general  description  is  here 
given  of  the  best  methods  of  making  meteorological  observations 
and  the  instruments  used  for  the  purpose.  This  is  prepared  specially 
for  the  information  of  the  voluntary  observers  of  the  United  States 
Weather  Bureau  under  the  Department  of  Agriculture. 

Observations  have  a  two-fold  object :  First,  to  obtain  a  knowledge 
of  the  climate  of  a  region,  that  is,  the  average  of  the  .various  air- 
conditions  over  a  country  that  affect  the  convenience,  comfort,  or 
health  of  the  people,  or  influence  plant  growth  ;  second,  the  dis- 
covery and  improvement  of  rules  and  methods  for  making  weather 
predictions.  For  this  purpose  the  observations  over  an  extensive 
area  of  country  are  used  to  trace  the  origin,  direction,  and  rate  of 
progress  of  pressure  and  temperature  changes  in  the  air,  the  occur- 
rence of  rainfall,  and  the  direction  and  velocity  of  wind  as  dependent 
on  the  variety  in  distribution  of  pressure  and  temperature  over  a 
region. 

The  successful  predictions  of  the  principal  weather  changes,  such 
as  cold  waves,  storm  winds,  heavy  rainfalls,  and  great  river  floods, 
are  of  the  highest  practical  value  to  people.  The  attainment  of  this 
important  object,  even  with  only  a  partial  degree  of  success,  is  a 
sufficient  motive  for  the  labor  bestowed  in  making  and  preserving 
the  vast  amount  of  weather  observations  now  going  on  in  most 
countries  all  over  the  world. 

Observations  are  desired  in  at  least  one  place  in  every  county  of 
every  State  in  the  United  States.  For  the  purpose  of  procuring 
these  observations  the  Chief  of  the  Weather  Bureau  is  authorized  by 
law  to  loan  instruments  to  observers  desirous  of  making  the  observa- 
tions, but  on  certain  conditions.  These  conditions  are  the  safe  keep- 
ing and  return  of  instruments,  and  that  a  copy  of  the  observations 
be  furnished  to  the  Weather  Bureau  Office,  Department  of  Agri- 
culture, at  Washington,  D.  C.,  free  of  expense  to  the  United  States 
Government. 

9 


i  fc-  10 

G'f fimiry"' precautions  "'arfe  required  in  the  use  of  instruments. 
Observers  are  not,  however,  held  responsible  for  the  breakage  or  loss 
of  instruments  due  to  unavoidable  accidents  incident  to  their  use  in 
the  making  of  observations. 

The  observations  in  which  the  Weather  Bureau  is  most  interested 
are  rainfall  and  temperature.  Thermometers  and  rain-gauges  are 
furnished  to  the  observers  free  of  any  charges  for  instruments  or 
for  transportation  to  the  observers. 

Blank  forms  and  franked  envelopes  are  furnished  to  the  observers 
for  transmission  of  reports  to  Washington,  D.  C.,  free  of  expense  to 
the  observer.  Observers  ajre  also  furnished  free  with  a  copy  of  the 
Monthly  Weather  Review,  an  official  publication  which  gives  a  sum- 
mary of  the  weather  all  over  the  United  States  for  each  month,  the 
mean  temperatures  and  total  rainfalls  for  places,  descriptions  of 
progress  of  principal  storms,  cold  waves,  and  floods  during  the 
month,  etc. 

Voluntary  observers  are  not  furnished  with  barometers,  wind- 
vanes,  or  anemometers.  Many  of  the  observers  have  these  instru- 
ments, their  own  private  property,  and  instructions  are,  therefore, 
here  given  regarding  their  use  and  care. 

Observations  of  air  pressure  and  wind  velocity  by  voluntary  ob- 
servers are  not  desired  by  the  Weather  Bureau.  Enough  data  of 
this  nature  for  the  purposes  of  the  Bureau,  the  forecasting  of  weather, 
are  obtained  from  the  regular  meteorological  stations.  Observa- 
tions of  pressure  by  voluntary  observers  are  mainly  of  interest  to 
the  observers  themselves  at  the  time  of  the  observation  in  esti- 
mating the  location  of  storm  centers  in  vicinity  where  there  is  no 
access  to  the  daily  weather  map  issued  by  the  Weather  Bureau.  The 
detailed  or  average  values  of  pressures  are  of  little  value  for  climate. 
The  rainfall  and  temperature  observations  are  intended  to  be  used 
in  the  Weather  Bureau  in  working  up  the  climatology  of  the  whole 
country. 

A  trustworthy  record  of  the  weather  is  always  of  interest  to  any 
community,  and  is  often  of  very  great  practical  value.  It  is  one  of 
the  objects  of  the  Weather  Bureau  to  foster  and  encourage  the  keep- 
ing of  such  records.  There  are  numerous  calls  for  records  of  the 
weather  as  evidence  in  courts  in  important  law  cases  months,  and 
even  years,  after  the  record  is  made.  Contractors  and  others  inter- 
ested in  outside  work  often  want  a  record  of  days  when  there  was 
rain  or  high  winds,  when  streams  were  frozen  over  or  swollen  with 
floods,  etc.  Farmers  are  interested  in  the  state  of  the  season, 
whether  forward  or  backward,  as  regards  temperature  and  rainfall. 
A  double  supply  of  blank  forms  is  furnished  observers  in  order  that 
a  copy  of  the  weather  record  may  be  retained  for  their  own  use. 

A  person  desiring  to  become  a  voluntary  observer  is  requested  to 
fill  out  a  form  (No.  4006),  a  specimen  of  which  is  shown  below. 


11 

[FORM  No.  4006— Mis.] 

APPLICATION  FOR  METEOROLOGICAL  INSTRUMENTS. 

I, ,  County  of ,  and  State  of ,  do  hereby  request  to  be 

furnished  with  one  maximum  thermometer,  one  minimum  thermometer,  and  one 
standard  rain-gauge,  with  the  necessary  supports,  for  use  in  making  daily  observa- 
tions of  temperature  and  precipitation,  and  Thereby  agree  to  send  a  monthly  report  of 
such  observations  on  Form  No.  1008,  Meteorological — blanks  and  penalty  envelopes  to  be 
furnished  by  the  Weather  Bureau — to  the  Observer,  Weather  Bureau, . 

I  do  further  agree  to  RETURN  all  of  said  instruments  to  the  Observer,  Weather 

Bureau, ,  or  to  turn  them  over  to  a  duly  appointed  successor,  whenever, 

from  any  cause,  except  sickness  or  unavoidable  absence  from  home,  I  fail  to  make  the 
observations  hereinbefore  agreed  upon. 

I  further  agree  to  exercise  a  reasonable  degree  of  care  in  handling  the  above- 
named  instruments,  to  care  for  them  as  I  would  my  own  property,  and  to  return  those 
which  may  be  accidentally  broken  or  otherwise  become  unserviceable,  with  a  state- 
ment covering  the  circumstances  which  caused  the  damage. 

(Signature) 

I,- ,  postmaster  of ,  do  hereby  certify  that ,  who 

signs  the  above  application,  is  personally  well  known  to  me,  and  I  fully  believe  him 
to  be  a  responsible  person,  and  that  he  will  faithfully  perform  the  obligations  assumed 
by  him  in  his  application. 

(Signature) 

Postmaster. 
(Date) ,  189    . 

U.  S.  DEPARTMENT  OF  AGRICULTURE, 

WEATHER  BUREAU, 

(STATION) 

Issued  maximum  thermometer  No. ,  minimum  thermometer  No. ,  rain- 
gauge  No. ,and  notified  the  central  office. 

Observer,  Weather  Bureau. 
(Date) ,  189    . 

The  observer  is  accepted  if  there  is  not  already  one  in  the  vicinity. 
A  list  of  the  various  observers  and  their  location  is  published  every 
three  months  in  the  Weather  Review. 

Voluntary  observers  receive  no  pay  for  services. 

.   METEOROLOGICAL  INSTRUMENTS. 

The  most  important  conditions  of  the  air,  the  subject  of  inves- 
tigation, and  the  instruments  used  in  observing  them  are  as  follows: 

Temperature  observed  by  means  of  thermometers. 

Water- vapor  pressure  in  air,  measured  with  dew-point  apparatus  or  psychrometer. 

Pressure,  observed  with  mercurial  barometer,  aneroid  barometer,  and  hypsometer. 

Rainfall,  measured  with  rain-gauge. 

Rainfall,  reaching  different  depths  in  the  earth,  measured  with  a  percolation 
gauge  or  lysimeter. 

Snowfall,  melted,  measured  with  a  rain-gauge. 

Wind  direction,  observed  by  a  wind  vane. 

Wind  velocity,  measured  by  an  anemometer. 

Clond  motion,  observed  with  nephoscope. 

Cloudiness,  amount  in  fractional  part  of  the  sky  covered,  estimated  with  the  eye. 

Sunshine  duration,  measured  with  a  sunshine  recorder. 

Electrical  potential,  measured  with  an  electrometer. 

Evaporation,  measured  with  evaporometer. 

Fog,  haze,  estimated. 


12 

In  river  observations  the  important  conditions  are: 

River  stages,  the  vertical   height  in  feet  of  river  surface  above  low  water, 
observed  on  a  river-gauge. 

River-current  velocity,  measured  with  a  current-meter  or  ship's  log. 

THERMOMETERS. 

The  most  important  condition  of  the  air  is  temperature.  A  mer- 
curial thermometer  used  in  observing-  temperature  consists  of  a  glass 
bulb  and  stem  containing  mercury.  Substances  expand  slightly  with 
heat.  Mercury  expands  seven  times  as  much  as  glass.  The  height 
of  the  top  of  column  of  mercury  in  the  stem  is  a  measure  of  the 
temperature.  The  thermometer  takes  the  temperature  of  the  air  by 
the  contact  of  particles  of  air  with  the  bulb. 

The  best  forms  of  thermometers  have  the  graduations  etched  on 
the  glass-stem  of  the  instrument. 

On  the  Fahrenheit  scale,  the  only  scale  in  use  in  meteorological 
observations  in  the  United  States,  the  temperature  of  the  melting 
point  of  ice  is  taken  as  32°,  usually  called  freezing  point,  and  the 
temperature  of  boiling  water  under  a  standard  pressure  of  the  air 
of  29.922  inches,  as  212°,  usually  called  boiling  point. 

The  temperature  of  melting  ice  varies  slightly  with  the  pressure 
of  the  air.  Doubling  the  pressure  of  the  atmosphere  lowers  the 
temperature  0.0135  ot  a  degree.  This  must  be  distinguished  from 
the  effect  of  an  additional  pressure  of  an  atmosphere  on  the  bulb  of 
a  thermometer.  For  an  ordinary  thermometer  the  increase  of  pres- 
sure on  the  bulb  for  one  atmosphere  compresses  the  bulb  to  such  an 
extent  that  the  column  of  mercury  in  the  stem  will  stand  0.5  of  a 
degree  higher  than  without  the  pressure. 

When  a  thermometer  is  made,  the  stem  is  closed  with  the  mercury 
near  the  top,  so  that  it  contains  very  little  air.  When  the  top  of  the 
stem  is  broken,  admitting  the  pressure  of  the  outside  air,  the  reading 
is  lowered  about  0.5  of  a  degree. 

At  boiling  point,  doubling  the  pressure  of  the  air  raises  the  boiling 
point  to  250°. 

The  volume  of  the  stem  from  32°  to  212°  is  divided  into  one- 
hundred  and  eighty  equal  parts  by  the  graduations  of  the  scale. 
The  temperature  corresponds  to  the  graduation  the  mercury  reaches 
on  stem  numbering  the  degrees  consecutively  from  32°  up,  and 
numbering  down  consecutively  below  32°. 

The  indications  of  a  mercurial  thermometer  are  trustworthy  as 
low  down  as  temperatures  on  the  very  verge  of  the  melting  point  of 
frozen  mercury,  —  37°-9.  Mercury  vaporizes  at  a  temperature  of 
642°.6. 

For  very  accurate  temperatures  several  requirements  are  neces- 
sary. The  thermometer  used  in  taking  the  temperature  should  be 
compared  with  a  standard  thermometer  by  comparative  readings  of 


13 

the  two  instruments  at  different  points  along  the  scale  in  water  kept 
thoroughly  stirred.  For  points  below  32°  the  comparisons  have  to 
be  made  in  a  bath  of  alcohol.  In  this  way  a  list  of  corrections  of 
the  thermometer  is  made,  which  are  to  be  applied  to  any  readings 
made  in  order  to  obtain  the  true  temperature. 
The  following  is  a  specimen: 


SCALE  READING. 

CORREOTIO: 

-28° 

-f  1°.0 

-18° 

+  0.     4 

—    8 

-0.  2 

4    2 

—  0.  6 

12 

—  1.1 

22 

—  0.  6 

32 

—  0.  4 

42 

-0.  3 

52 

-0.  1 

62 

+  0.  1 

72 

+  0.  3 

82 

+  0.  5 

92 

+  0.  8 

102 

-fl.  1 

The  correction  should  be  applied  algebraically  to  the  scale  read- 
ing of  the  thermometer.  For  a  temperature  not  given  in  the  list 
of  scale  readings,  an  interpolated  value  of  the  correction  is  used. 
For  the  reading  of  67°.5  for  instance,  the  correction  is  +  °-2  and 
the  true  temperature  670.7;  for  22°  the  correction  is  —  0.6  and  the 
true  temperature  21°. 4. 

Caution  is  required  in  applying  the  correction  to  a  minus  scale 
reading  when  the  temperature  is  below  zero.  For  the  reading  of 

—  28°.o,  for  instance,  the  correction  is  +  i.o;  the  true  temperature  is 

—  28.0  4-  i.o  =  —  27.0.      In  applying  the  correction,  like  signs  add, 
dissimilar  signs  subtract. 

There  is  a  constant  rise  in  the  freezing  point  of  a  thermometer 
with  age,  due  to  a  slow  and  steady  contraction  of  the  glass  bulb.  A 
thermometer  graduated  the  day  it  is  filled  will  read  a  degree  and  a 
half  higher  a  week  after.  Good  thermometers  are  not  graduated 
until  a  year  or  two  after  filling.  In  the  course  of  three  years  the 
freezing  point  will  rise  a  half  degree.  This  varies  very  much  in 
different  instruments,  depending  on  the  nature  of  the  glass.  Some 
varieties  of  glass  are  now  used  in  making  thermometers  that  vary 
very  little  with  age. 

Any  change  in  the  freezing  point  of  a  thermometer  changes  the 
whole  series  of  corrections  throughout  the  scale  correspondingly. 
In  the  case  of  the  thermometer  above,  if  the  correction  at  freezing 
point  is  found  on  packing  the  instrument  in  melting  ice  at  some 
future  time  to  be  —0.9  of  a  degree  instead  of  —0.6;  that  is,  if  its 
reading  in  ice  is  320.9  instead  of  320.6  as  would  be  the  case  now,  then 
there  is  a  change  of  0.3  of  a  degree  throughout  the  scale,  the  instru- 


14 

ment  reading  that  much  higher  at  every  point.  The  correction  at 
62°  instead  of  being  —  o.i  will  be  —  0.4  at  72°,  instead  of  -f  o.i;  it 
will  be  —  0.2,  etc. 

After  a  thermometer  has  been  heated  to  a  very  high  temperature, 
as  the  boiling  point,  there  is  a  depression  of  the  freezing  point  of 
about  0.4  of  a  degree,  which  is  fully  recovered  in  six  weeks.  Sub- 
jecting a  thermometer  to  a  very  high  temperature,  as  500°,  raises  the 
freezing  point  permanently  from  10°  to  1 8°.  Long  exposure  to  a  low 
temperature  as  —  30°  will  raise  the  freezing  point  about  o.i  of  a 
degree. 

The  valid  freezing  point  correction  to  be  used  in  correcting  the 
indications  of  a  thermometer  to  the  true  temperature  is  the  freezing 
point  correction  observed  just  after  exposure  of  the  thermometer  to 
the  temperature.  Ordinarily  for  such  range  as  there  is  in  the 
temperature  of  the  air  the  freezing  point  observed  once  a  year  gives 
the  correction  with  sufficient  accuracy  to  o.i  of  a  degree. 

Thermometers  are  made  with  cylindrical  and  spherical  bulbs. 
The  cylindrical  form  is  the  most  popular  at  the  present  time.  It  is 
usually  more  sensitive  than  a  spherical  bulb,  as  it  exposes  more 
surface  to  the  air  for  the  same  quantity  of  mercury  contained. 
Spherical  bulbs  can,  however,  be  blown  thinner  than  cylindrical 
ones,  and  the  glass  is  commonly  more  uniform  in  thickness. 

In  observations  of  rapid  change  of  temperature,  as  in  a  balloon 
ascent,  a  thermometer  has  to  be  very  sensitive  to  give  a  correct 
result.  In  fact  a  correction  has  to  be  applied  where  accuracy  is 
required.  A  thermometer  may  sometimes  be  found  in  error  from  a 
break  in  the  column  of  mercury  or  from  a  bubble  of  air  in  the  bulb. 

A  thermometer  to  be  in  the  best  condition  ought  not  to  have  its 
column  broken  when  held  with  the  bulb  uppermost.  To  remove 
the  air  the  speck  in  the  bulb  should  be  first  brought  to  the  junction 
of  the  stem  and  bulb  by  jarring  the  latter  in  the  open  palm  of  the 
hand.  When  the  speck  is  at  the  junction,  prepare  ice  or  snow  or 
cold  water  and  cool  the  bulb  down,  then  invert  it,  and  all  the  mercury 
will  run  to  the  other  end,  or  may  be  made  to  do  so  by  a  gentle  jar, 
Now  heat  the  bulb,  either  in  the  hand  or  at  a  long  distance  from  a 
heated  surface,  being  careful  to  hold  the  stem  horizontal  or  inclined, 
with  the  bulb  end  up.  This  will  cause  the  mercury  to  rise  and  drive 
the  air  before  it.  Care  should  be  taken  never  to  heat  so  much  that 
the  top  of  the  column  is  less  than  a  half-inch  from  the  end  of  the 
other  detached  column,  as  otherwise  the  thermometer  may  be  broken. 
If,  now,  the  thermometer  is  held  vertically  the  two  columns  will  join, 
or  may  be  made  to  do  so  by  a  gentle  jar  in  the  open  palm  of  the  hand; 
the  air  speck  will  be  in  the  tube,  but  the  mercury  will  join  by  a 
slender  thread  alongside  of  the  speck.  Now,  if  the  bulb  be  cooled, 
the  air-speck  will  remain  stationary,  but  the  mercury  will  flow  past 
it  toward  the  bulb. 


15 

By  repeating  the  process  the  speck  may  be  finally  driven  into  the  top 
of  the  tube,  where  it  will  do  no  harm.  In  cooling  off  the  thermometer 
for  the  last  time  care  should  be  taken  that  the  speck  in  the  column 
be  at  a  point  before  inversion  which  is  above  the  temperature  of  the 
cooling  liquid,  otherwise  the  column  left  after  cooling  down  will  be 
so  short  that  it  cannot  be  driven  down  on  inversion.  If  it  be  found 
impossible  to  get  the  short  column  down  on  inversion  after  cooling, 
it  may  be  easily  lengthened  by  jarring  the  bulb  in  the  open  hand, 
holding  the  bulb  down.  In  case  there  is  much  air  separating  the 
two  columns  it  will  be  found  impossible  to  unite  them  when  the 
upper  column  is  quite  short,  the  air-speck  being  so  large  that  the 
thread  of  mercury  cannot  pass  it.  In  such  case  it  may  be  necessary 
to  heat  the  upper  part  of  the  tube  quite  hot  where  there  is  no  mer- 
cury. This  expands  the  air  and  brings  a  slight  pressure  to  bear  upon 
the  top  of  the  mercury.  If  the  thermometer  while  still  hot  be  jarred 
upon  the  open  hand  the  columns  will  frequently  unite.  If  all  other 
efforts  fail  take  the  tube  from  the  brass  scale  and  gradually  heat  the 
part  above  the  main  column  in  an  alcoliol  flame,  by  running  it  back 
and  forth  in  the  flame  and  twirling  it  continually  in  the  fingers,  in 
order  to  heat  it  uniformly. 

The  mercury  will  be  volatilized  and  scattered  along  the  bore. 
Then  by  heating  up  the  thermometer  the  main  column  will  take  up 
the  various  detached  portions.  Some  thermometers  are  provided 
with  an  expansion  chamber  at  the  top  of  the  tube,  and  in  such  case 
it  is  only  necessary  to  heat  the  bulb  until  the  air-speck  has  been 
driven  into  the  chamber.  After  that,  by  holding  the  bulb  down,  on 
cooling  it  will  be  found  that  the  air  has  remained  behind  and  there 
is  a  continuous  column  of  mercury.  In  the  latter  kind  of  thermom- 
eter it  will  frequently  be  found  that  a  portion  of  the  mercury  has 
lodged  in  the  chamber,  and  is  entirely  separated  from  the  column 
in  the  tube.  In  such  case  a  slight  jar  will  sometimes  send  the 
mercury  down.  If  this  does  not  avail  then  the  bulb  should  be 
warmed  till  the  column  in  the  tube  reaches  the  mercury  in  the 
chamber.  When  this  is  done,  after  cooling  it  will  be  found  that  the 
column  is  perfect.  Sometimes  the  chamber  becomes  completely 
filled,  and  no  effort  can  dislodge  the  mercury.  In  such  case  it  is 
necessary  to  heat  the  chamber  in  an  alcohol  flame,  being  careful  to 
turn  the  tube  in  the  hand  in  order  to  apply  the  heat  uniformly.  In 
a  short  time  the  murcury  will  be  expanded  sufficiently  to  flow  down 
the  tube  on  its  being  held  with  the  bulb  down.  A  flame  should 
never  be  brought  into  contact  with  a  thermometer  bulb. 

MINIMUM    THERMOMETERS. 

For  temperatures  below  the  freezing  point  of  mercury  alcohol 
thermometers  are  used,  standardized  by  comparison  with  a  gas  ther. 
mometer  at  low  temperatures.  The  alcohol  minimum  thermometer 


16 

is  the  only  trustworthy  form  of  minimum  thermometer  for  observ- 
ing the  lowest  temperature  reached  by  the  air. 

Alcohol  thermometers  are  not  as  accurate  as  mercurial.  The 
alcohol  wetting  the  glass  surface  makes  the  reading  unequal  at 
different  times.  Even  with  the  greatest  care  an  accuracy  of  0.6  of  a 
degree  is  the  best  that  can  be  attained.  The  errors  are  liable  to  be 
much  larger  at  times  on  account  of  air  bubbles  forming  in  the  column 
or  part  of  the  alcohol  evaporating  and  lodging  in  the  upper  part  of 
stem. 

An  index  half  an  inch  long  made  of  enamel  is  fitted  loosely  in  the 
bore  of  the  stem  and  immersed  in  the  liquid.  When  the  temperature 
falls  the  index  is  carried  along  the  bore  and  the  top  stops  at  the 
lowest  point  reached  by  the  top  of  the  alcohol  column.  The  reading 
of  the  top  of  the  index  gives  the  lowest  temperature.  It  is  reset  for 
another  observation  by  raising  the  bulb  end  of  the  thermometer, 
which  causes  the  index  to  slide  down  the  bore  until  it  stops  at  the 
end  of  the  column  of  alcohol. 

The  minimum  thermometer  is  set  nearly  horizontal,  with  the  bulb 
end  lower  than  the  top.  Jarring  by  the  wind  is  apt  to  displace  the 
index  and  make  it  read  too  low. 

Bubbles  are  apt  to  develop  in  the  bore  or  bulb,  and,  making  a  break 
in  the  continuity  of  the  alcohol  column,  make  the  top  read  too  high. 

An  instrument  must  be  carefully  examined  from  time  to  time  for 
the  formation  of  alcohol  in  the  bore  above  the  column,  which  causes 
the  instrument  to  read  too  low. 

Instructions. — The  many  different  ways  in  which  the  alcohol  becomes 
separated  make  it  impossible  to  unite  it  by  any  single  method, 
different  methods  being  required  not  only  for  different  conditions, 
but  also  for  different  thermometers. 

Frequently  there  are  only  a  few  short  detached  portions  near  the 
top,  and  the  index  slides  freely  along  the  lower  portion  of  the  tube 
and  drops  into  the  bulb.  Again,  the  detached  columns  are  sometimes 
found  all  along  the  tube,  and  the  index  is  caught  and  held  at  some 
point  above  the  main  column.  In  such  a  case  it  is  advisable  to  first 
bring  the  index  into  the  bulb,  as  follows: 

First  process.— Hold  the  thermometer  lightly  between  the  thumb 
and  fingers  and  strike  the  lower  end  of  the  metallic  scale  against  the 
top  of  a  table,  or  other  firm  object;  first,  however,  interposing  one 
or  two  thicknesses  of  cloth,  or  several  folds  of  paper,  so  as  not  to 
produce  too  severe  a  shock  upon  the  thermometer.  The  taps  of  the 
thermometer  should  be  made  light  at  first,  and  the  index  examined 
to  see  if  it  has  not  moved  along  the  tube  even  a  little  distance,  as 
can  be  told  by  noticing  the  exact  position  of  the  index  in  reference 
to  the  graduations  on  the  tube.  If  several  taps  fail  to  move  the 
index,  increase  the  force  of  the  taps,  a  little  at  a  time,  until  the  index 
starts,  after  which  repeat  the  operation  until  the  index  gets  within 


17 

the  continuous  column.  Here  it  will  fall  of  its  own  weight  into  the 
bulb.  Generally,  this  will  be  all  that  is  necessary  to  place  the  index 
in  the  bulb.  Sometimes  the  detached  columns  will  also  have  been 
partly  or  wholly  united.  If  the  column  is  still  broken  in  places  the 
observer  should  try  a  few  more  taps  and  quickly  examine,  very  care- 
fully, if  very  small  portions  of  the  alcohol  cannot  be  seen  slowly 
moving  along  the  sides  of  the  tube  toward  the  main  column.  If  this 
is  the  case  a  continuation  of  the  taps  will  unite  the  columns;  on  the 
other  hand,  and  in  case  the  index  cannot  be  made  to  move  with  quite 
hard  taps,  it  is  advisable  to  try  some  of  the  methods  described  next, 
being  careful  always  to  avoid  carrying  any  process  so  far  as  to 
endanger  breaking  the  thermometer. 

Second  process. — Grasp  the  thermometer  securely  a  little  below  the 
middle,  with  the  bulb  end  down,  and  strike  the  edge  of  the  metal 
back  opposite  the  broken  column  sharply  against  the  fleshy  portion 
of  the  palm  of  the  other  hand,  or,  if  necessary,  against  a  small  block 
of  wood  held  in  the  hand.  A  continued  jarring  in  this  way  often 
causes  the  alcohol  to  run  down,  though  in  many  cases  a  large  number 
of  taps  are  necessary.  Observers  should,  therefore,  not  give  up  if 
the  column  does  not  unite  at  once,  but  should  watch  very  closely  for 
the  movements  of  small  portions  of  alcohol  along  the  sides  of  the 
tube.  Here,  again,  care  must  be  exercised  not  to  strike  too  hard, 
and  to  hold  the  thermometer  by  the  metal  back  in  such  a  manner  as 
not  to  squeeze  or  press  against  the  stem  of  the  thermometer  itself. 

When  the  bore  of  the  thermometer  is  large  the  above  process  is 
almost  sure  to  unite  the  column.  Good  results  are  also  obtained 
with  thermometers  of  fine  bore,  though  the  latter  often  require  some 
time  if  the  column  is  badly  scattered. 

Third  process. — This  method  can  also  be  used  in  place  of  those 
above,  and  is  sometimes  effective  in  forcing  the  index  into  the  bulb. 
Grasp  the  thermometer  a  little  above  the  middle,  holding  it  horizon- 
tally, clasping  the  fingers  and  hand  firmly  against  the  edges  of  the 
metallic  back,  but  not  so  as  to  bring  any  pressure  upon  the  glass  tube, 
which  should  be  turned  toward  the  person  and  with  the  bulb  toward 
the  front.  With  the  thermometer  in  this  position  and  about  as  high 
as  the  head,  and  the  arm  free  from  the  body,  quickly  lower  the  arm 
and  hand  a  foot  or  more,  turning  the  wrist  at  the  same  time,  so  that 
the  bulb  of  the  thermometer  describes  a  somewhat  circular  path 
downward  through  the  air,  stopping  the  motion  with  a  sudden  jerk 
just  as  the  thermometer  is  vertical.  If  the  thermometer  is  grasped 
properly  a  very  violent  motion  can  be  given  in  this  way  without 
danger.  It  will  sometimes  be  necessary  to  repeat  the  operation  a 
large  number  of  times  to  entirely  unite  the  detached  columns. 

When  all  the  bubbles  have  been  removed  from  the  alcohol  column, 
the  observer  can  then  see  how  the  instrument  works.  Hold  the  ther- 
mometer vertically  and  warm  up  the  bulb  by  holding  it  in  the  hand; 


18 

then  turn  the  instrument  upside  down.  Watch  the  index  as  it  glides 
along  the  tube ;  when  it  strikes  the  top  of  the  column  it  will  at  once 
stop.  This  operation  of  bringing  the  index  to  the  top  of  the  column 
is  called  " setting"  the  thermometer.  Next  hold  the  thermometer 
horizontally  and  remove  the  hand  from  the  bulb.  As  the  latter  cools 
off  the  index  will  be  dragged  backwards  toward  the  bulb,  but  always 
remains  at  the  end  of  the  column  of  alcohol.  It  is  a  good  plan  to 
hasten  the  cooling  by  placing  a  little  wet  cloth  or  piece  of  ice  against 
the  bulb.  When  you  have  watched  the  index  go  down  with  the 
column,  warm  the  bulb  again  with  the  hand.  The  column  will  go 
up  immediately,  but  the  alcohol  will  flow  around  the  index  and  leave 
it  at  the  lowest  point.  That  is,  the  index  remains  so  that  its  top  end 
is  at  the  lowest  point  reached  by  the  alcohol  column,  and  the  mini- 
mum temperature  is  indicated  in  this  way.  The  thermometer  must 
be  held  horizontally  throughout  these  operations. 

When  the  thermometer  is  not  in  use  for  observation  it  is  a  good 
plan  to  hang  it  up,  as  bubbles  are  less  likely  to  form  in  the  tube  in 
this  position. 

MAXIMUM    THERMOMETERS. 

For  observing  the  highest  temperatures  occurring  during  the  day 
a  maximum  thermometer  is  used.  The  form  of  maximum  in  use  by 
the  Weather  Bureau  is  a  mercurial  thermometer  with  a  contraction 
in  the  bore  of  the  tube  near  the  bulb. 

When  cooling  of  the  instrument  sets  in  the  thread  of  mercury  in 
the  bore  separates  from  the  rest  at  the  point  of  contraction,  leaving 
the  top  of  the  column  near  the  highest  temperature  reached.  It  is  reset 
for  another  observation  by  whirling  it  on  a  pivot  at  the  top  of  the 
metal  strip  to  which  the  tube  is  attached.  The  centrifugal  force 
developed  drives  the  detached  thread  back  to  the  main  body  of 
mercury. 

When  a  maximum  thermometer  is  not  read  for  several  hours  after 
the  highest  temperature  has  occurred  and  the  air  in  the  meantime 
has  cooled  down  15°  or  20°,  the  highest  temperature  indicated  by 
the  top  of  the  detached  thread  of  mercury  may  be  too  low  by  half  a 
degree  from  the  contraction  of  the  thread. 

When  the  fall  of  temperature  from  the  highest  point  is  very  slow 
a  little  of  the  mercury  may  pass  down  before  the  thread  breaks, 
especially  when  there  is  no  wind  to  cause  a  slight  jarring  of  the 
instrument. 

In  some  instruments  the  narrowing  of  the  bore  develops  a  strong 
capillary  action,  and  when  the  connection  of  the  column  is  broken 
the  detached  thread  jumps  up  a  little,  causing  it  to  register  a  little 
too  high.  This  is  most  apt  to  occur  in  very  cold  weather  when  the 
detached  columns  are  very  short.  It  also  occurs  with  longer 
detached  columns  when  the  thermometer  is  nearly  horizontal. 


19 


The  cut  below  shows  the  appearance  of  the  maximum  and  mini- 
mum thermometers  when  properly  attached  to  the  wooden  board 
and  supports  furnished  with  the  thermometers. 

The  upper  thermometer  is  the  mini- 
mum. The  lower  thermometer  is  the 
maximum.  All  new  maximum  and  min- 
imum thermometers  have  their  respective 
names  stamped  upon  the  metallic  backs. 

The  maximum  thermometer  is  always 
rilled  with  mercury,  sometimes  called 
quicksilver.  All  maximum  thermome- 
ters have  a  round  brass  hub-shaped  pro- 
jection fastened  to  the  top  of  the  back. 

The  most  distinctive  peculiarity  of  the 
maximum  thermometer,  however,  is  not 
so  easily  seen  as  those  mentioned  above. 
If  observers  will  hold  the  thermometer 
vertically,  with  the  bulb  down,  they  will 
r-;  notice  that  the  merciirial  column  does  not 
g  extend  entirely  into  the  bulb.  A  very 
close  examination  of  the  thermometer  at 
the  point  just  above  the  bulb,  where  the 
mercurial  column  stops,  will  show  that 
the  glass  tube  seems  to  have  been  squeezed 
almost  together  at  that  point,  as  appears 
in  the  above  drawing. .  This  is  just  what 
has  been  done,  and  the  passage-way  for 
the  mercury  is  so  fine  at  that  point  that 
the  mercury  will  only  go  through  with 
some  difficulty.  The  observer  should  next 
hold  his  fingers  around  the  bulb.  Pres- 
ently the  mercury  in  the  bulb,  as  it  be- 
comes warm,  will  be  forced  to  pass  the 
constricted  portion  of  the  tube,  flowing 
through  in  little  spurts.  When  the  fingers 
are  removed  from  the  bulb  the  mercury 
below  the  constriction  withdraws  toward 
the  bulb,  but  none  of  the  mercury  above  the  constriction  can  get 
back  and  the  column  remains  just  the  same  length  as  it  was  when 
the  bulb  was  warmest.  In  this  way  the  maximum  temperature 
is  registered. 

Observers  sometimes  think  the  above-described  constriction  in 
the  tube  is  a  defect,  but  such  is  not  the  case,  and  no  effort  should 
be  made  to  cause  the  column  to  completely  unite  at  this  portion  of 
the  tube. 

1)377—3 


20 

Some  forms  of  "exposed"  thermometers  look  like  maximum  ther- 
mometers, but  observers  should  never  have  the  least  difficulty  in 
telling  one  from  the  other.  The  maximum  thermometers  used  by 
the  Weather  Bureau  always  have  the  constriction,  and  only  maxi- 
mum thermometers  have  the  "hub"  at  the  top  of  the  metallic  back. 

If  it  is  desired  to  set  the  maximum  thermometer  before  placing 
in  position  on  the  supports,  this  may  be  done  by  following  the  in- 
structions given  in  the  "third  process"  for  minimum  thermometers, 
by  which  means  the  column  of  murcury  will  be  driven  back  into 
the  bulb  and  the  instrument  "set"  at  the  current  temperature. 

INSTRUCTIONS   FOR   SETTING  UP  MAXIMUM   THERMOMETER. 

For  the  support  of  the  maximum  thermometer,  the  long  brass  pin 
with  a  nut  must  be  screwed  into  the  board  in  the  hole  to  the  extreme 
right.  The  nut  must  be  taken  off  and  the  pin  slipped  through  the 
hole  in  the  upper  end  of  the  instrument,  and  then  secured  by  replac- 
ing the  nut  and  screwing  it  tight.  The  plain  brass  pin  must  then 
be  inserted  in  the  hole  on  the  left  of  the  board,  and  the  thermom- 
eter placed  upon  it,  so  as  to  slightly  incline  in  that  direction.  The 
end  of  the  pin  to  which  the  maximum  thermometer  is  attached  must 
be  occasionally  oiled  to  prevent  friction.  The  maximum  thermom- 
eter is  read  by  observing  the  number  of  degrees  upon  the  scale  at  the 
top  of  the  column  of  mercury.  After  having  taken  a  reading,  the  ob- 
server must  remove  the  pin  at  the  left  and  then  take  hold  of  the 
thermometer  about  3  inches  from  the  top  and  spin  it  ar^tmd 
several  times  in  the  direction  of  the  motion  of  the  hands  of  a  watch, 
until  the  bulb  and  stem  below  the  constriction  are  filled  with  mer- 
cury. Care  must  be  taken  not  to  touch  the  bulb,  and  also  that  the 
nut  is  screwed  up  sufficiently  tight  to  prevent  the  instrument  from 
striking  against  the  side  of  the  board  to  which  it  is  fastened.  After 
adjustment,  gradually  raise  the  instrument  to  a  nearly  horizontal 
position  and  insert  the  pin  as  before.  Care  must  be  taken  in  ele- 
vating the  instrument  not  to  raise  the  bulb  too  high,  as  the  column 
of  mercury  would  then  break  and  run  to  the  upper  end  of  the  tube. 

INSTRUCTIONS   FOR  SETTING   UP  MINIMUM   THERMOMETER. 

The  brass  support  (a  long,  thin,  narrow  piece  of  curved  brass, 
containing  two  screw-holes  near  the  center),  to  which  the  minimum 
thermometer  is  attached,  will  be  secured  to  the  board  in  the  follow- 
ing manner  : 

The  first  screw-hole  will  be  made  8  inches  from  the  inner  edge  of 
the  bevel  at  the  end  of  the  board  on  the  right,  and  $/&  inch  below 
the  inner  edge  of  the  bevel  on  the  top  of  the  board. 

After  securing  the  brass  support  to  the  board  with  the/™/  screw, 
attach  the  thermometer  by  screwing  it  to  the  support,  and  let  the 
lower  end  of  the  thermometer-scale  rest  in  the  notch  cut  in  brass 


21 

support  to  the  left.  Then  bring  the  left  end  of  the  support  to  such 
a  position  that  the  upper  edge  of  the  thermometer-scale  will  run 
exactly  parallel  with  the  edge  of  the  board.  The  second  screw  will 
then  be  inserted,  which  will  secure  the  brass  support  firmly  to  the 
board. 

The  minimum  thermometer,  when  attached  to  the  support  in  this 
manner,  will  then  be  exactly  horizontal,  which  position  is  best 
adapted  to  prevent  any  movement  of  the  index  not  caused  by  the 
actual  changes  in  temperature.  The  top  of  the  thermometer  must 
be  fastened  by  the  small  brass  screw  upon  the  support  and  the 
lower  end  dropped  into  the  notch  to  the  left,  after  having  been 
raised  sufficiently  to  allow  the  small  index  in  the  tube  to  move 
down  to  the  top  of  the  column  of  spirit.  This  instrument  is  read  by 
observing  the  number  of  degrees  on  the  scale  where  the  top  of  the  index 
rests. 

If,  at  any  time,  the  column  should  be  found  to  be  broken  and 
bubbles  are  observed  in  the  tube,  the  instrument  should  be  taken 
off  the  supports  and  an  effort  made  to  unite  the  column,  as  given 
above. 

To  "  set  "  the  minimum  thermometer  after  a  reading  has  been 
made,  the  bulb,  or  lower  end  of  the  thermometer,  must  be  elevated 
a  few  inches  until  the  index  moves  down  to  the  end  of  the  column 
of  alcohol,  and  then  carefully  lowered  into  the  notch  at  the  left  of 
the  brass  support,  as  before. 

After  resetting  the  maximum  and  minimum  thermometers,  the 
top  of  the  mercurial  column  of  the  maximum  and  the  top  of  the 
alcohol  column  of  the  minimum  should  read  alike. 

Example :  The  above  illustration  represents  the  thermometers 
just  after  they  have  been  set.  It  will  be  seen  that  the  top  of  the 
mercurial  column  of  the  maximum  indicates  a  reading  of  89°.  The 
top  of  the  alcohol  column  and  the  top  of  the  index  of  the  minimum 
show  the  same  reading,  89°.  This  indicates  that  the  two  instru- 
ments have  been  properly  set. 

The  instruments  must  be  set,  in  the  manner  described,  once 
each  day,  immediately  after  taking  the  observation.  In  no  case 
must  they  be  set  at  any  other  time. 

The  wooden  boxes  and  packing  material  received  with  these  ther- 
mometers should  be  carefully  preserved  for  use  in  returning  instru- 
ments. 

In  packing  thermometers  for  transportation  each  instrument  must 
be  carefully  wrapped  in  sheet  cotton;  the  whole  surrounded  with 
excelsior,  or  other  packing  material,  in  the  wooden  box,  and,  to  pre- 
vent shifting,  all  spaces  should  be  filled  with  cotton.  The  lids  of  the 
wooden  boxes  should  always  be  secured  with  screws,  as  the  jarring 
resulting  from  driving  nails  into  the  wood  is  liable  to  break  the 
thermometer. 


22 


THERMOMETER    SHELTER. 

To  take  the  temperature  of  the  air  the  thermometers  are  set  up 
inside  of  a  cubical  wooden  lattice  work  inclosure  called  a  shelter. 
To  get  the  temperature  of  the  air  accurately  a  thermometer  must  be 
protected  from  the  direct  rays  of  the  sun,  which  cause  it  to  read  too 
high.  Without  a  covering  in  the  night  time  by  radiation  to  the  sky 
a  thermometer  is  apt  to  read  too  low.  The  ground  is  cooler  than  the 
air  at  night,  but  the  effect  of  ground  radiation  on  a  thermometer  is 
inappreciable  at  a  greater  height  than  4  feet  above  the  ground. 

A  thermometer  in  quiet  air  in  the  sunshine  may  read  10°  or 
more  higher  than  the  air  temperature,  depending  on  the  strength 
of  the  wind  prevailing  at  the  time.  With  the  air  perfectly  calm 
it  may  read  even  20°  higher.  With  a  wind  of  twenty  miles 
an  hour  it  will  read  only  a  few  tenths  of  a  degree  higher  than  the 
temperature  in  the  shade,  which  is  the  true  temperature  of  the  air. 
Moving  particles  of  air  coming  continually  in  contact  with  the  bulb 
rapidly  carry  away  the  heat  received  from  the  sun.  When  there  is 
no  wind  a  thermometer  bulb  creates  a  local  atmosphere  of  warm  air 
around  it.  In  a  vacuum  where  all  contact  with  air  is  shut  off,  a 
thermometer  in  the  sunshine  becomes  greatly  heated.  A  thermom- 
eter in  the  sunshine  whirled  rapidly  will  read  about  half  a  degree 
higher  than  the  true  air  temperature. 

The  reading  of  a  thermometer  in  the  sunshine  in  nearly  quiet  air 
is  not  a  meteorological  datum  of  any  value. 

The  form  of  shelter  shown  below  and  described,  has  been  found 
after  years  of  experience  to  be  the  best  adapted  for  the  purpose. 


fi" 

8* 

r^^ 

6" 

8" 

36 

DJ 

36" 



B_ 

42 


36' 


42" 


Section  thr<f 
Scale  1=50"  Scale  1  =  5 


4" 

Section,  thro*  A.B 
Scale  2*5" 


FlG.  2. — DESIGN  FOR  THERMOMETER   SHELTER. 

The  frame  is  cubical,  3  feet  on  a  side.     The  bottom  is  of  close 
boards,  pine,  %  inch  thick.     The  roof  is  double,  sloping  8  inches 


-•23 

from  front  to  rear,  with  6  inches  between.  The  high  or  front  side 
is  made  to  face  north.  The  front  has  a  door  15  inches  wide  the 
whole  length  of  side,  and  opens  down.  The  sides  are  of  blind  work; 
the  slats  of  white  pine  3^  inches  wide  and  ^  inch  thick,  slop- 
ing at  an  angle  of  45°  to  the  horizontal.  The  distance  from 
center  to  center  is  i%  inches.  Each  slat  has  a  strip  ^  inch  wide 
and  X  inch  thick  nailed  along  its  length  on  the  inside  flush 
with  the  bottom.  The  frame  is  in  four  parts  and  furnished  with 
screws  for  putting  together.  The  bottom  has  one  piece  loose  and 
hinged,  so  as  to  allow  of  dropping  down,  and  a  button  to  hold  it  up. 
Inside  there  is  a  frame  for  holding  the  thermometers,  consisting  of 
two  upright  strips  i  by  3  inches  fastened  to  long  strips  screwed  to 
the  right  and  left  rails  of  the  frame  with  a  strip  i  by  2^2  inches  be- 
tween the  thermometers.  It  is  painted  white.  This  shelter  should 
be  exposed  upon  a  free  grass  plat  where  there  is  no  obstruction  to 
the  wind,  and  at  a  height  of  at  least  TO  feet  above  the  sod.  An  expo- 
sure from  the  roof  of  a  building  is  not  objectionable,  provided  a  plat- 
form 20  feet  square  be  first  laid  and  the  shelter  be  raised  8  feet 
above  the  roof.  The  good  ventilation  of  a  roof  is  especially  valua- 
ble. An  exposure  4  feet  above  sod,  especially  among  trees,  or  where 
there  is  very  little  natural  ventilation,  is  objectionable,  from  the 
fact  that  under  such  conditions  radiation  from  the  sod  into  space 
abnormally  cools  the  sod,  and  that  in  turn  cools  the  air  above  it. 
This  effect  is  especially  active  at  and  a  few  hours  after  sunset.  The 
expense  of  such  an  exposure  as  that  just  described  may  deter  many 
observers  from  adopting  it,  and  in  such  case  the  following  substitute 
is  suggested  : 

WINDOW    OR    WALL    SHELTER. 

As  its  name  implies  this  may  be  used  from  the  iraside  of  a  house 
or  it  may  be  placed  upon  a  wall  and  read  from  the  outside  ;  in  either 
case  the  protection  for  the  thermometers  is  the  same  and  may  be 
very  easily  prepared  by  any  one.  Ordinarily  a  wall — which  should 
invariably  be  one  looking  due  north  or  only  a  few  degrees  to  the 
east  or  west  of  north — will  give  too  low  a  temperature  by  day,  except 
possibly  in  winter,  and  always  too  high  a  temperature  by  night. 
The  reason  for  this  is  not  far  to  seek.  During  the  daytime  a  large 
wall  does  not  gain  the  temperature  of  the  free  air  but  is  nearly 
always  below  it,  while  the  reverse  is  true  at  night. 

These  conditions  are  specially  prominent  in  the  case  of  a  large 
brick  or  stone  wall.  The  two  effects  above  may  be  partly  counter- 
balanced by  the  following  considerations  :  First,  the  effect  of  the 
coolness  of  the  wall  by  day  may  be  offset  by  heat  reflected  and  radi- 
ated from  other  walls  or  surroundings;  second,  the  warmth  at  night 
may  be  offset  in  like  manner  by  radiation  to  the  sky.  This  will 
readily  suggest  a  form  of  exposure  adopted  by  many,  one  very  easy 


24 

to  manage,  and  being  the  least  expensive  of  any.  Select  a  north 
window,  preferably  of  an  unoccupied  room,  especially  in  winter. 
Fasten  the  blinds  open  at  right  angles  to  the  wall  of  the  house  in 
order  to  shield  from  the  effects  of  the  'direct  solar  radiation  on  the 
north  side  both  morning  and  evening.  Fasten  a  narrow  strip,  per- 
haps 3  inches  wide,  across  the  window  outside  and  from  8  to  12 
inches  from  the  window  pane ;  to  this  fasten  the  thermometers. 
Some  difficulty  will  be  experienced  in  setting  the  maximum  ther- 
mometer and  in  wetting  the  wet  bulb  if  the  room  is  occupied,  as  the 
heated  air  in  winter  will  disarrange  the  readings  of  the  self-regis- 
tering thermometers.  This  may  be  partially  obviated  by  opening 
the  upper  window  and  conducting  all  operations  through  it.  In  this 
case  the  heated  air  rises  and  does  not  affect  the  readings.  If  there  is 
no  wind,  or  if  the  wind  is  south,  the  greatest  disadvantage  arising 
from  this  exposure  will  be  the  lack  of  ventilation  of  the  wet  bulb. 
This  may  be  partly  obviated  by  working  a  fan  hinged  near  the  wet 
bulb  and  operated  by  a  handle  in  the  room. 

(1)  The  most  important  consideration  is  as  full  and  free  natural 
ventilation  as  is  possible.     This  is  essential  even  if  an  artificial  ven- 
tilation be  employed  for  the  dry  and  wet  bulbs.     Since  the  maximum 
and  minimum  temperatures  are  necessarily  unventilated,  the  shelter 
itself  should  give  accurate  results  for  these. 

(2)  Provision  should  be  made  to  avoid  effects  of  direct  solar  radi- 
ation, and  also  of  heat  reflected  and   radiated   from   surrounding 
objects. 

(3)  Rain  should  be  excluded,  but  only,  as  far  as  possible,  in  con- 
nection with  (i).     During  a  rain-storm  the  dry  and  wet  bulbs  must 
necessarily  give  the  same  values,  the  air  being  nearly  saturated; 
hence  a  wetting  at  this  time  is  not  harmful. 

(4)  All  perraanent  heating  effects  from  warm  walls,  chimneys,  etc., 
should  be  carefully  avoided. 

DAILY  MEAN  TEMPERATURE. 

The  mean  or  average  daily  temperature  at  a  place  is  the  mean  of 
the  twenty-four  observations  made  at  a  place  at  every  hour  of  the 
day.  Hourly  observations  of  temperature  are  made  at  only  a  few 
places  over  the  world.  Approximate  values  of  the  mean  daily  tem- 
perature are  obtained  by  various  short  processes. 

The  mean  of  the  highest  and  lowest  temperature  of  the  day,  as 
observed  with  a  maximum  and  minimum  thermometer,  gives  a  tem- 
perature on  the  average  about  i°.o  higher  throughout  the  year 
than  the  hourly  mean  at  places  in  the  United  States.  This  is  the 
mean  adopted  by  the  Weather  Bureau  for  the  observations  at  the 
regular  meteorological  stations.  In  England  the  same  mean  gives 
the  temperature  0.6  of  a  degree  too  high  throughout  the  year,  being 
1.2  too  high  in  August  and  0.3  too  high  in  January. 


25 

One-half  the  sums  of  the  temperatures  at  9  o'clock,  morning  and 
evening  local  times,  gives  the  mean  too  small  by  about  0.5  of  a  degree. 

One-half  the  8  o'clock,  morning  and  evening,  gives  the  tempera- 
ture in  the  United  States  only  a  few  tenths  of  a  degree  below  the 
mean  of  the  day. 

When  it  is  possible  to  observe  the  temperature  at  7  a.  m.,  2  p.  m., 
and  9  p.  m.,  a  very  close  approximation  to  the  mean  hourly  tempera 
ture  of  the  day  can  be  obtained  by  taking  one  fourth  of  the  sum  of 
the  temperatures  at  7  a.  m.,  2  p.  m.,  and  twice  the  9  p.  m. 
Example:  7  a.  m.  +  2  p.  m^+  2  (9  p.  m.) 


This  combination  is  recommended  where  the  observers  can  con- 
veniently make  the  observations  at  those  hours. 

SOLAR    RADIATION    THERMOMETER. 

A  solar  radiation  thermometer  consists  of  a  mercurial  thermometer 
inclosed  in  a  glass  tube  from  which  the  air  has  been  exhausted. 

They  are  commonly  used  in  pairs,-one  with  a  black  bulb,  the  other 
bright,  and  are  exposed  side  by  side  in  the  sun.  They  are  known 
as  the  bright  and  black  bulb  in  vacuo.  The  black  bulb  will  read  in 
the  sunshine  from  30°  to  60°  higher  than  the  temperature  of  the  air 
in  the  shade.  The  temperature  attained  depends  on  the  rate  at 
which  heat  is  being  received  from  the  sun  by  the  bulb  and  radiated 
to  the  glass  inclosure  surrounding  it,  which  is  nearly  the  tempera- 
ture of  the  air. 

The  thermometer  shown  in  Fig.  3,  is  usually  made  as  a  maximum 
with  constriction  in  bore  of  stem.  The  inclosing  bulb  is  about  1.5 
inches  in  diameter.  The  highest  temperature  occuring  during  the 
day  is  registered  by  the  instrument,  and  it  is  reset  each  day  for 
another  observation. 


FIG.  3.  FIG.  4. 

The  difference  of  bright  and  black  bulb  shows  the  difference  in 
the  heating  effect  of  the  sun  on  the  air  at  different  times.     When 


26 

there  is  a  great  deal  of  moisture  or  dust  suspended  in  the  air  more 
of  the  sun's  heat  is  absorbed  in  passing  through  it  than  when  there 
is  very  little.  The  more  the  air  is  heated  the  less  the  bulb.  It  is 
not  a  good  measure,  however,  as  the  difference  in  the  bulbs  depends 
on  so  many  things  besides  the  clearness  of  the  air. 

The  thickness  of  the  film  of  blacking  on  the  bulb  affects  the  dif- 
ference; also  the  size  of  the  bulb  the  thickness  of  the  glass  bulb  of 
the  inclosure,  and  variations  in  the  nature  of  the  glass  not  percep- 
tible to  the  eye;  also  the  depth  of  air  through  which  the  sun's  rays 
come.  Some  varieties  of  glass  transmit  more  of  the  sun's  heat  than 
others.  A  great  deal  too  depends  on  the  perfection  of  the  vacuum. 
The  presence  of  the  least  trace  of  air  depresses  the  reading.  Instru- 
ments the  same  to  all  appearances,  when  exposed  side  by  side,  will 
differ  as  much  as  6°  in  the  temperatures  indicated. 

Nothing  of  any  importance  has  yet  been  derived  from  observations 
of  these  instruments.  No  way  has  yet  been  devised  of  making 
different  pairs  comparable. 

At  high  altitudes,  as  on  mountain  tops,  the  black  bulb  in  sunshine 
may  read  as  high  as  the  boiling-point  of  water. 

In  the  arctic  regions,  in  summer,  the  difference  between  the  bright 
and  black  is  sometimes  as  great  as  96°. 

The  name  radiation  thermometer  is  also  applied  to  an  alcohol 
thermometer  exposed  in  the  open  air  on  the  ground  in  the  night 
time  to  get  the  lowest  radiation  temperature  of  the  soil.  The  ther- 
mometer shown  in  figure  4  is  the  same  as  the  ordinary  minimum, 
except  that  there  is  a--protecting  glass  tube  over  the  graduations  to 
shield  the  graduation  of  the  scale  from  the  action  of  the  dew  and 
prevent  its  being  washed  out.  During  the  night  in  still  air  it  may 
take  on  a  temperature  10°  lower  than  that  of  the  air  a  few  feet  above 
the  ground. 

When  a  thermometer  is  wetted  repeatedly  or  put  in  alcohol  or 
ether,  the  graduation  becomes  indistinct  or  illegible  from  the  marks 
in  the  etching  washing  out.  To  replace  the  marking,  artist's  black 
pigment  is  used.  It  should  be  rubbed  on  the  thermometer  when  the 
tube  is  quite  dry,  and  the  tube  then  wiped  with  glazed  paper  to 
remove  the  pigment  not  in  marks. 

THERMOSCOPE. 

A  sealed  glass  tube  containing  an  alcohol  solution  of  camphor 
gum  is  sometimes  used  as  an  indicator  of  temperature  changes. 
When  the  temperature  is  high  the  camphor  is  all  dissolved  and  the 
liquid  is  clear.  At  a  low  temperature  the  camphor  crystallizes  out, 
forming  a  fleecy  whitish  looking  mass.  A  thermoscope  of  this  kind 
is  sometimes  sold  mounted  on  the  same  board  with  a  thermometer, 
and  is  often  erroneously  supposed  to  indicate  changes  in  pressure, 
or  the  electrical  condition  of  the  air. 


27 

WATER  VAPOR  IN  AIR. 

The  air  contains  vapor  of  water  transparent  and  colorless  like  its 
other  gaseous  components.  It  only  becomes  visible  on  condensing 
to  fog  or  cloud,  which  is  only  water  in  a  fine  state  of  division,  the 
particles  varying  in  diameter  from  0.0006  to  0.0050  of  an  inch.  The 
amount  is  very  variable  at  different  times,  even  in  the  vicinity  of 
the  ocean,  depending  mainly  on  the  temperature  of  the  air.  On 
very  cold  days  it  forms  no  more  than  the  one-thousandth  part  of  the 
air.  On  very  warm  days  it  may  be  as  great  as  one-fiftieth  part. 

The  amount  of  moisture  chat  can  exist  as  vapor  in  the  air  depends 
on  the  temperature.  There  is  a  certain  pressure  of  vapor  corre- 
sponding to  every  temperature  which  cannot  be  exceeded  ;  beyond 
this  there  is  condensation.  This  temperature  is  called  the  tem- 
perature of  saturation  for  the  pressure.  When  the  temperature  of 
the  air  diminishes  until  the  saturation  temperature  for  the  vapor  con- 
tained is  reached,  any  further  fall  causes  a  condensation  of  moisture. 

The  temperature  at  which  this  occurs  at  any  time  is  called  the 
dew-point  temperature  of  the  air  at  that  time.  The  less  the  quantity 
of  moisure  the  air  contains  the  lower  will  be  the  temperature  of  its 
dew  point. 

This  property  of  vapor  is  made  use  of  to  determine  the  pressure 
of  vapor  contained  in  the  air  at  any  time.  The  number  of  degrees 
the  dew  point  is  below  the  temperature  of  the  air  is  called  the 
depression  of  the  dew  point. 

The  relation  of  water- vapor  pressure  and  temperature  has  been 
very  thoroughly  investigated  by  skillful  experimenters  on  account 
of  its  importance  in  the  theory  of  the  steam  engine. 

For  different  saturation  temperatures  the  vapor  pressures,  in 
inches  of  mercury,  and  the  weight  of  vapor,  in  grains,  contained  in  a 
cubic  foot  of  air  are  as  follows  : 

Temperature  Vapor  pres-  Weight  in  a 

of  satura-  sure    in  cubic  foot, 

tion.  inches.  grains. 

0°  0. 038  0. 56 

10  0. 063  0. 87 

20  0. 103  1. 32 

30  0. 164  1. 96 

40  0. 246  2. 85 

50  0. 360  4. 08 

60  0. 517  5. 74 

70  0.732  7.98 

80  1.022  10.93 

90  1. 408  14. 79 

100  1. 916  19. 77 

The  quantity  of  water  contained  in  the  air  is  nearly  proportional 
to  the  vapor  pressure.  The  air  is  never  perfectly  saturated,  not 
even  when  rain  is  falling  ;  neither  is  it  ever  perfectly  dry  at  any 
place. 

The  number  expressing  the  vapor  pressure  in  the  air  at  any  time 
divided  by  the  number  expressing  the  pressure  of  saturation  for  the 


28 

temperature  prevailing  at  the  time  is  the  fraction  of  saturation,  and 
is  called  the  relative  humidity.  It  is  expressed  in  hundredths  of 
the  pressure  at  saturation  and  is  called  percentage  of  relative 
humidity.  With  the  air  at  a  temperature  of  60°,  for  instance,  if  the 
pressure  of  vapor  contained  is  found  to  be  0.420  of  an  inch,  the 
pressure  at  that  temperature  for  saturation  being  0.5 17,  the  percent- 
age of  relative  humidity  is  the  one  divided  by  the  other,  expressed 
in  hundredths  as  the  unit,  or  81  per  cent. 

Relative  humidity  expresses  relative  amount  of  moisture  in  the 
air  only  as  long  as  the  temperature  of  the  air  remains  constant. 
For  this  reason  relative  humidity  is  an  imperfect  datum.  At  a  low 
temperature  even  a  high  relative  humidity  represents  a  very  small 
amount  of  vapor  actually  in  the  air,  while  a  low  relative  humidity 
at  a  high  temperature  represents  a  great  deal.  At  a  temperature 
of  30°,  for  instance,  a  relative  humidity  of  90  per  cent,  corresponds 
to  0.149  °f  an  inch  vapor  pressure,  while  50  per  cent,  at  80°  corre- 
sponds to  a  pressure  of  0.511,  or  more  than  three  times  as  much  as  is 
contained  at  the  temperature  of  30°. 

The  amount  of  moisture  in  the  air  is  ascertained  indirectly  by 
observing  the  temperature  at  which  dew  is  deposited,  or  by  deter- 
mining the  vapor  pressure  by  means  of  the  difference  in  indication 
of  a  dry  and  wet  bulb  thermometer. 

PSYCHROMETER. 

Two  thermometers,  one  with  wet  and  the  other  with  dry  bulb, 
when  used  to  determine  the  vapor  pressure  in  the  air,  is  called  a 
psychrometer.  The  wet  bulb  consists  of  an  ordinary  mercurial 
thermometer  with  a  wrapping  of  muslin  on  the  bulb  kept  saturated 
with  water.  The  water  evaporating  from  the  muslin  cools  the  bulb. 
The  dryer  the  air  the  greater  the  amount  of  evaporation  and  the 
more  the  bulb  is  cooled,  and  consequently  the  reading  of  wet  bulb 
is  lower  the  dryer  the  air. 

From  a  series  of  comparisons  of  the  indications  of  a  dew-point 
apparatus  and  a  psychrometer  the  relation  between  the  pressure  of 
the  vapor  in  the  air  and  its  dependence  on  the  difference  between 
the  reading  of  the  dry  and  wet  bulb  thermometer  has  been  ascer- 
tained. Tables  showing  this  relation  are  given  here.  With  the  differ- 
ence of  dry  and  wet  bulb  and  the  temperature  of  the  air,  the  reading 
of  the  dry  bulb,  the  temperature  of  dew  point  of  the  air  is  obtained 
from  one  table  and  the  relative  humidity  from  the  other. 

The  result  found  for  vapor  pressure  with  a  psychrometer  varies  a 
little  in  still  air,  depending  on  whether  the  wet  bulb  is  spherical  or 
cylindrical.  It  also  depends  on  the  absolute  size  of  the  bulb  and 
whether  the  stem  above  the  bulb  for  an  inch  or  so  is  wrapped  with 
the  wet  muslin.  All  of  these  effects  are  obviated  by  whirling  the 
psychrometer  so  as  to  cause  a  brisk  renewal  of  air  around  the  bulb. 


29 

When  whirled  before  making  the  readings,  instruments  of  the  most 
diverse  patterns  are  strictly  comparable.. 

Where  the  highest  accuracy  is  required  the  most  minute  correc- 
tions of  the  thermometers  have  to  be  applied.  Small  errors  in  the 
observed  temperatures  affect  the  deduced  vapor  pressures  very  much. 
The  tables  given  are  based  on  psychrometer  readings  uncorrected 
for  the  difference  of  temperature  between  the  bulb  of  the  wet  ther- 
mometer and  its  stem,  the  stem  being  at  the  temperature  of  the  air 
while  the  bulb  is  a  variable  number  of  degrees  lower,  depending  on 
the  dryness  of  the  air.  For  a  long  stem  this  correction  may  amount 
to  0.3  or  0.4  of  a  degree. 

SLING    PSYCHROMETER. 

For  values  of  dry  and  wet  bulb  temperatures  the  most  accurate 
method  yet  devised  consists  in  the  use  of  dry  and  wet  bulb  ther- 
mometers fastened  together  and  whirled  quite  rapidly.  Any  one 
may  very  readily  prepare  this  instrument.  The  thermometers  may 
be  fastened  back  to  back  or  side  by  side,  the  wet  bulb  an  inch  or  so 
lower  than  the  dry;  then  a  rather  strong  wire  or  a  good  many  folds 
of  a  thinner  one  should  be  passed  through  the  hole  at  the  top  of  the 
scale,  and  to  this  a  stout  cord  about  12  inches  long  should  be  fastened. 
A  glove  finger  will  serve  to  protect  the  finger  while  whirling.  The 
method  of  use  is  very  simple.  First  wet  the  muslin  by  immersion 
and  wait  about  a  minute  till  the  wet  bulb  has  fallen  somewhat  (this 
is  specially  necessary  in  a  very  dry  air,  as  otherwise  the  muslin  may 
become  dry  before  it  has  reached  its  lowest  reading),  then  whirl  the 
thermometers  about  50  times,  stop  and  read  the  wet  bulb,  then  whirl 
20  times  and  again  read.  If  the  temperature  of  the  wet  bulb  has  not 
changed  it  may  be  considered  as  the  correct  value.  If  there  has  been 
a  change  to  a  lower  reading  continue  the  alternate  whirling  and 
reading  until  the  wet  bulb  reaches  its  lowest  reading  or  one  slightly 
higher  than  the  one  previous.  The  lowest  reading  is  to  be  recorded. 
A  velocity  of  12  feet  per  second  is  ample. 

Great  care  should  be  taken  never  to  allow  the  muslin  to  become 
dry  or  even  partly  so.  At  night  the  whirling  may  be  done  at  any 
convenient  spot  where  there  is  a  good  ventilation.  The  temperature 
should  be  taken  at  least  6  feet  above  sod  or  at  any  height  greater 
than  that.  If  there  is  a  wind  blowing  the  observer  should  stand  to 
leeward;  if  there  is  no  wind,  in  cool  weather,  it  may  be  necessary  to 
walk  about  while  whirling  in  order  to  overcome  any  possible  vitiation 
of  the  reading  from  the  heat  of  the  body.  When  the  sun  is  shining,  in 
the  daytime,  the  temperature  should  be  taken  in  the  shade  of  a 
small  object,  high  tree,  or  even  umbrella,  but  not  in  the  shade  of  a 
wall  or  large  house.  The  use  of  this  instrument  when  the  wet  bulb 
is  covered  with  ice  is  specially  satisfactory.  The  lowest  reading  at 
any  time  may  be  obtained  in  three  minutes,  while  with  a  rather 


30 

close  window  shelter,  when  the  air  is  still,  it  will  require  a  delay  of 
an  hour  tor  even  an  approximate  value. 

HAIR  HYGROMETER. 

A  human  hair  when  freed  of  oil,  by  soaking  in  ether  for  twenty- 
four  hours,  has  the  property  of  changing  by  about  one-thirtieth  part 
of  its  length  between  a  very  dry  and  very  moist  condition  of  the  hair 
at  ordinary  temperatures. 

A  hair  hygrometer  consists  of  a  hair  about  12  inches  in  length 
mounted  in  a  brass  frame,  the  lower  end  wound  once  around  a  pulley 
and  stretched  by  a  weight  of  about  i  gramme,  or  15.4  grains  Troy. 

As  the  pulley  turns  with  the  varying  length  of  hair  dependent  on 
the  dryness  of  the  air,  an  attached  index  moves  over  a  graduated  arc, 
indicating  the  percentage  of  humidity  or  relative  humidity.  It  is 
graduated  with  reference  to  a  dew-point  apparatus  or  a  psychrometer. 
It  is  not  an  accurate  instrument,  its  results  being  complicated  with 
changes  of  temperature.  It  is  only  used  in  closed  spaces  where 
other  means  of  observing  vapor  pressure  are  not  available.  It  gives 
a  rough  result  very  quickly.  It  is  sometimes  used  for  humidity  ob- 
servations at  temperatures  below  zero. 

BAROMETER. 

The  air  presses  on  everything  at  the  surface  of  the  earth  with  a 
pressure  equal  to  the  weight  of  the  column  of  air  above  it  to  the 
limit  of  the  atmosphere.  This  pressure  is  on  the  average  about  14.67 
pounds  to  the  square  inch,  and  corresponds  to  the  pressure  of  a 
column  of  mercury  about  30  inches  in  height.  The  pressure  of  the 
air  is  continually  varying  within  small  limits.  Variations  in  pres- 
sure are  not  noticeable  to  the  senses  except  in  the  case  of  sudden 
great  changes. 

The  barometer  is  an  instrument  used  to  measure  the  pressure  of 
the  air.  Pressures  are  expressed  in  the  equivalent  height  of  a 
column  of  mercury  reduced  to  the  temperature  of  freezing  point. 

The  barometer  is  a  glass  tube  open  at  one  end  and  closed  at  the 
other,  about  38  inches  long  and  %  inch  in  interior  diameter.  The 
open  end  dips  into  the  cistern  of  the  instrument  containing  mer- 
cury which  extends  inside  the  tube  up  to  a  height  corresponding 
to  the  pressure  of  the  air.  The  tube  above  the  column  of 
mercury  contains  no  air.  The  vertical  height  of  the  level 
of  the  top  of  mercury  column  above  the  level  of  mercury  in 
the  cistern,  in  inches  and  parts  of  an  inch,  is  called  the  barometer 
reading.  This  height  is  constantly  varying  within  small  limits  with 
the  changing  pressure  of  the  air. 

The  barometer  is  first  set  up  by  the  maker  by  filling  the  tube 
with  mercury  and  inverting  it  with  the  open  end  immersed  in  the 
mercury  in  the  cistern,  the  end  being  closed  temporarily  with  the 
gloved  finger  until  it  is  immersed  in  the  mercury.  When  the  finger 


31 


a 


is  removed  the  mercury  runs  down  the  tube,  oscillating  back  and 
ferth  and  finally  coming  to  rest  at  a  height  corresponding  to  the 
pressure  of  the  air  outside. 

The  observation  of  pressure  consists  in  ascertaining  the  height  of 
column  at  any  time  by  means  of  the  scale  attatched  to  the  instru- 
ment. For  this  purpose  an  ivory  point,  the  lower  extremity  of 
which  is  the  zero  of  the  scale,  is  fixed  to  the  frame-work  of  the 
instrument,  on  the  upper  portion  of  which  is  the  graduated  scale 
and  vernier.  The  mercury  in  the  cistern  is  brought  in  contact  with 
the  ivory  point  and  the  vernier  of  the  scale  is  adjusted  to  the  top  of 
the  column  ;  the  reading  of  the  scale  will  then  give  the  height  of 
the  column  of  mercury. 

Placing. — The  barometer  should  be  placed  in  a  room  of  a  tempera- 
ture as  uniform  as  possible,  and  not  exposed  to  the  sun.  It  must 
be  suspended  so  that  the  top  of  the  column  will  be  at  the  height  of 
the  eye,  near  a  window,  in  such  a  manner  as  to  be  lighted  perfectly 
without  exposure  either  to  the  direct  rays  of  the  sun  or  to  the  cur- 
rents of  the  air  which  are  always  to  be  found  at  the  joinings  of  the 
windows.  When  the  barometer  has  to  be  fixed  to  the  wall,  as  is  the 
case  with  all  the  self-recording  and  some  other 
barometers,  care  must  be  taken  to  secure  the  tube 
(in  a  position  perfectly  vertical,  regulating  it  by  the 
plumb-line,  first  in  the  front,  then  at  the  sides,  in 
two  vertical  planes  cutting  each  other  at  right  angles. 
When  the  instrument  is  so  constructed  as  to  take 
its  equilibrium  itself,  as  the  Fortin  barometers  and 
those  of  J.  Green,  it  is  enough  to  hang  it  on  a  strong 
hook.  These  conditions  being  fulfilled,  the  rest  of 
the  arrangement  may  be  varied  according  to  the 
nature  of  the  localities.  For  the  Fortin  and  Green 
barometers  the  following  arrangement  is  convenient, 
and  may  be  almost  everywhere  adopted.  (See  Fig.  5.) 
A  small  elongated  box  (a  b),  some  inches  longer  than 
the  barometer  and  a  little  broader  than  its  cistern,  is 
firmly  set  against  the  wall  (w  w'),  near  the  window, 
in  such  a  manner  as  to  open  in  a  direction  parallel 
to  the  panes  ;  at  the  summit  (a)  it  has  a  strong  hook 
(h  /$'),  which  extends  beyond  the  box  about  2  or  3 
inches,  and  on  which  the  barometer  is  suspended. 
The  instrument  remains  generally  in  the  box,  which 
is  closed  by  a  movable  cover,  and  which  protects  it 
from  external  injuries,  from  dust,  and  from  the  direct 
radiation  of  warm  bodies,  or  the  currents  of  air  from 
F  5  the  window,  and  diminishes  the  effect  of  the  too 

sudderfvari-ations  of  temperature.  When  it  is  to  be  observed,  the 
barometer  is  taken  by  the  upper  end  of  the  tube,  and  the  suspending 


32 

ring-  is  made  to  slide  toward  the  end  of  the  hook.  The  instrument 
is  then  in  the  full  light  of  the  window,  in  front  of  which  the  observer 
places  himself ;  the  summit  of  the  mercurial  column,  as  well  as  the 
surface  of  the  mercury  in  the  cistern,  are  completely  lighted,  and 
the  reading  becomes  easy  and  certain.  Moreover,  the  slight  oscillat- 
ing movement  impressed  on  the  instrument  by  changing  its  place 
breaks  the  adherence  of  the  mercury  to  the  glass,  and  thus  prepares 
a  good  observation.  After  the  reading,  the  barometer  is  again 
slipped  gently  into  the  box,  and  this  is  closed. 

It  has  been  found  that  even  with  the  exercise  of  care  the  barom- 
eter cistern  strikes  against  the  box  when  it  is  pushed  in,  and  in  con- 
sequence the  instrument  deteroriates  quite  rapidly.  To  avoid  all 
possibility  of  injury  from  this  cause  it  is  only  necessary  to  make  two 
oblong  sets  of  openings  in  the  back  of  the  box,  one  just  at  the  ivory 
point  about  3  inches  long,  and  the  other  at  the  vernier  some  8  inches 
long,  and  place  in  these  plates  of  glass.  If  now  the  barometer  box 
be  suspended  vertically  in  a  north  window  it  will  be  a  very  simple 
matter  to  make  readings  by  opening  the  box  and  adjusting  the 
mercury  to  the  ivory  point  and  the  vernier  to  the  top  of  the 
column,  without  moving  the  barometer. 

Observation. — Note  the  degree  and  the  tenths  of  degrees  of  the 
thermometer  attached  to  the  instrument;  for  it  will  be  seen  that  the 
heat  of  the  observer's  body  soon  makes  it  rise. 

Incline  the  instrument  gently,  so  as  to  render  the  mercurial  column 
very  movable;  then,  after  having  restored  it  to  rest,  strike  several 
slight  blows  upon  the  casing,  in  such  a  manner  as  to  impress  on  the 
mercury  gentle  vibrations.  The  adherence  of  the  mercury  to  the 
glass  will  thus  be  destroyed,  and  the  column  will  take  its  true 
height  corresponding  to  the  outside  pressure  of  the  air. 

Bring,  by  means  of  the  adjusting  screw  at  the  bottom,  the  surface 
of  the  mercury  to  the  zero  of  the  scale. 

In  the  barometers  with  an  ivory  point,  as  the  Fortin,  Newman, 
and  Green  barometers,  the  extremity  of  this  point  is  the  zero  of  the 
scale,  which  must  be  brought  into  exact  contact  with  the  surface  of 
the  mercury.  This  takes  place  when  the  point  coincides  exactly 
with  its  image  reflected  below  by  the  mercury.  This  method  is  very 
good  when  the  surface  of  the  mercury  is  perfectly  pure  and  brilliant. 
It  is  generally  dimmed  by  a  slight  layer  of  oxide,  which  makes  the 
coincidence  of  the  point  with  its  image  uncertain.  It  is  safer  to 
judge  of  the  contact  in  a  different  manner.  From  the  moment  when 
the  point  does  more  than  touch  the  surface,  it  forms  around  itself, 
by  capillary  action,  a  small  depression,  which,  breaking  the  direction 
of  the  reflected  rays,  becomes  immediately  very  easy  to  discover. 
It  is  enough,  then,  to  raise  the  mercury  so  as  slightly  to  immerse  the 
point;  then  to  lower  it  gradually  iintil  the  little  depression  dis- 
appears. If  care  is  taken  to  make  a  good  light  fall  on  that  portion 


33 

of  the  mercury  which  is  under  the  point,  and  to  use  the  aid  of  a 
magnifier,  the  adjustment  of  the  point  thus  made  becomes  not  only 
easy,  but  very  certain,  and  the  errors  to  which  we  are  liable  are 
almost  insensible,  for  they  do  not  exceed  two  or  three  hundredths 
of  a  millimeter,  or  a  thousandth  of  an  inch. 

The  level  being  thus  adjusted  to  the  zero  of  the  scale,  proceed  to 
observe  the  height  of  the  summit  of  the  column.  Take  hold  of  the 
instrument  with  the  left  hand,  above  the  attached  thermometer, 
without  moving  it  from  the  vertical;  tap  it  gently  in  the  neighbor- 
hood of  the  top  of  the  column;  then,  by  means  of  the  screw,  lower 
the  slide  which  carries  the  vernier,  until  the  plane  passing  through 
the  two  lower  opposite  edges  of  it  is  exactly  tangent  to  the  summit 
of  the  meniscus — that  is,  the  convexity  which  terminates  the  column. 
This  is  the  case  when,  placing  the  eye  exactly  at  the  height  of  the 
summit  of  the  column,  it  is  seen  without  there  being  any  trace  of 
light  between  the  summit  and  the  edge  of  the  ring.  To  be  certain 
that  the  barometer  has  remained  quite  vertical  during  its  operation, 
leave  it  to  itself,  and,  when  it  is  at  rest,  look  again  to  see  whether  the 
ring  has  remained  tangential  to  the  summit  of  the  column.  If  it 
has  not,  the  verticality  has  been  disturbed;  it  must  be  adjusted 
anew.  It  is  necessary,  at  the  same  time,  to  examine  if  the  adjust- 
ment of  the  surface  of  the  mercury  in  the  cistern  has  remained  the 
same. 

Nothing  more,  then,  remains  than  to  read  the  instrument.  In  the 
English  barometers  the  inches  and  tenths  of  inches  are  read  directly 
on  the  scale,  the  hundredths  and  thousandths  on  the  vernier.  In 
the  French  barometers,  with  the  metrical  scale,  the  centimeters  and 
millimeters  are  read  on  the  scale,  and  the  fractions  of  millimeters 
on  the  vernier. 

THE    VERNIER. 

The  vernier  is  a  contrivance  for  measuring  fractional  portions  of 
one  of  the  equal  spaces  into  which  a  scale  or,  as  commonly  known, 
a  limb  is  divided.  We  will  call  the  fixed  portion  of  the  barometer, 
which  is  divided  into  equal  parts,  the  scale,  and  the  part  which 
moves  upon  it  by  means  of  the  screw  and  ratchet,  the  vernier.  A 
simple  inspection  will  indicate  the  nature  of  the  spaces  or  equal 
parts  of  the  scale.  In  the  common  barometer  there  are  inches  and 
tenths,  or  centimeters  and  millimeters.  Ordinarily  the  spaces  on 
the  whole  length  of  the  vernier  are  one  more  than  on  the  part  of  the 
scale  which  it  covers  :  /.  <?.,  if  the  vernier  has  ten  spaces  the  scale 
has  nine  in  the  same  length.  Determine  the  value  of  the  smallest 
space  on  the  scale  (in  the  common  Green  barometer  this  is  one- 
tenth  inch),  divide  this  by  the  number  of  spaces  on  the  vernier,  and 
we  get  immediately  the  least  count  of  the  vernier.  We  have  then  the 
following  rule  :  The  least  count  of  a  vernier  is  equal  to  one  of  the 


34 

spaces  of  the  scale  divided  by  the  number  of  equal  parts  on  the 
vernier. 

The  reading  of  the  instrument  depends  on  the  distance  from  the 
zero  point  or  the  beginning  of  the  scale  to  the  zero  point  of  the 
vernier.  If  we  take  a  vernier  and  make  its  zero  point  coincide  with 
one  of  the  divisions  on  the  scale  we  shall  find  the  second  line  exactly 
the  least  count  of  the  vernier  below  the  second  line  of  the  scale;  or, 
in  other  words,  if  we  move  the  vernier  until  its  second  line  coincides 
with  the  one  just  above  the  first  line  on  the  scale,  just  referred  to, 
we  shall  move  the  zero  just  the  least  count  above  its  first  position. 
In  the  same  way  the  zero  of  the  vernier  may  be  made  to  coincide 
with  other  divisions  on  the  limb,  and  the  distance  moved  will  depend 
on  the  number  of  the  line  on  the  vernier  above  its  zero  which  is 
coincident  with  the  line  on  the  scale.  We  have  then  the  simple  rule 
for  reading  a  vernier.  Read  the  scale  up  from  its  zero,  or  in  the 
direction  in  which  the  figures  on  the  graduations  increase  up  to  that 
line  of  the  scale  just  below  the  zero  of  the  vernier.  Call  this  the 
reading  of  the  scale.  Find  the  number  of  the  line  on  the  vernier, 
counting  the  first  line  above  the  zero  as  i,  which  coincides  most 
nearly  with  aline  on  the  scale;  multiply  the  least  count  of  the  vernier 
by  this  number;  this  will  give  the  reading  on  the  vernier;  the  sum 
of  these  two  readings  is  the  reading  of  the  instrument. 

The  observer  is  advised  to  test  his  ability  to  read  the  vernier  by 
setting  it  at  various  points  and  then  reading.  After  a  time  one 
reads  the  vernier  naturally,  without  being  obliged  to  follow  any 
fixed  rules.  It  will  seldom  occur  that  a  line  on  the  vernier  exactly 
coincides  with  one  on  the  scale,  and  after  a  little  practice  one  will 
be  able  readily  to  split  the  least  count;  for  example,  if  any  two  con- 
tiguous lines  of  the  vernier  lie  between  and  exactly  equidistant  from 
two  on  the  scale,  the  least  count  will  be  exactly  halved.  It  is  a  very 
good  plan,  in  order  to  avoid  serious  mistakes,  at  first  to  estimate  the 
position  of  the  zero  line  of  the  vernier  on  the  scale;  this  will  give 
the  approximate  value  of  the  reading  and  will  check  the  reading 
made  from  the  vernier. 

To  read  the  vernier,  we  must  look  out  for  the  line  that  coincides 
with  one  of  the  divisions  of  the  scale.  The  number  of  this  division 
of  the  vernier,  proceeding  from  zero,  indicates  the  number  of  tenths 
of  millimeters,  or  of  hundredths  of  an  inch,  which  must  be  added  to 
the  whole  number  given  by  the  scale.  If  none  of  the  divisions  of 
the  scale  coincides  exactly,  we  estimate  by  the  eye,  in  decimals,  the 
quantity  by  which  the  vernier  must  be  lowered  to  obtain  a  coinci- 
dence, and  this  is  added  to  the  fraction  already  obtained.  This  will 
be  hundreths  of  millimeters  in  the  metrical  barometer,  and  thou- 
sandths of  inches  in  the  English  barometers. 

The  following  figures  will  serve  as  examples;  the  instrument  is 
an  English  barometer: 


35 


In  Fig.  6  the  regulating  line,  which  is  the  lower  edge  of  the  vernier 
ring,  coincides  exactly  with  the  line  of  30  inches  on  the  scale.  The 
zero  and  the  tenth  division  of  the  vernier  are  also  in  exact  coinci- 
dence; that  is  to  say,  there  is  no  fraction.  We  shall  read  then  30.000 
inches. 

In  Fig.  7  the  regulating  line  does  not  fall  upon  any  of  the  divisions 
of  the  scale,  but  between  29l%  and  29-^-  inches.  There  is  then  a 
fraction  which  must  be  read  on  the  vernier.  Seeking  which  of  these 
divisions  coincides  with  that  of  the  scale,  we  find  that  it  is  the  fifth; 
we  shall  write  then  29.250  inches. 


-10 


-10 


80- 


FIG.  6. 


FIG.  7. 


FIG.  8. 


In  Fig.  8  we  see  that  the  height  falls  between  30  inches  and 
inches;  no  line  of  the  vernier  coincides  exactly;  but  the  line  7  is  a 
little  above,  the  line  8  is  a  little  below,  one  of  the  lines  of  the  scale; 
the  fraction  falls,  then,  between  seven  and  eight  hundredths.  Esti- 
mating in  tenths  the  distance  the  vernier  passes  over  between  the 
coincidence  of  7  and  that  of  8,  the  tenths  of  a  hundredth,  or  the 
thousandths  are  obtained.  In  the  latter  case  the  distance  above  7  is 
greater  than  the  half;  it  will  read  then,  greater  than  30.075  and 
less  than  30.080,  or  about  30.077.  It  will  always  be  easy  to  judge 
whether  the  top  approaches  nearer  the  upper  coincidence  than 
the  lower  coincidence;  in  the  former  case  the  fraction  is  greater 
than  .005;  in  the  latter  it  is  smaller  than  .005.  The  error  which  will 
be  committed  in  this  estimate  will  remain  less  than  .005;  after 
a  little  practice  it  will  rarely  exceed  .002,  always  supposing  the  scale 
is  well  graduated.  For  this  reading,  as  well  as  for  the  others,  it 
is  particularly  important  to  have  the  eye  exactly  at  the  height  of 

the  line  to  be  determined. 
9377 3 


36 

During  the  whole  time  of  the  observation  of  the  barometer  the 
observer  must  endeavor  to  protect  it  as  much  as  possible  from  the 
heat  which  radiates  from  his  body.  But  the  best  way  is  to  learn  to 
observe  rapidly.  All  the  operations  take  longer  to  describe  than  to 
execute;  one  or  two  minutes,  if  the  instrument  be  in  place,  three 
minutes,  if  it  is  to  be  taken  from  its  case  and  put  back  again,  are 
sufficient  for  a  practiced  observer  to  make  a  good  observation. 

The  barometer  consists  of  a  brass  tube  (Fig.  9)  terminating  at 
top  in  a  ring  A,  for  suspension,  and  at  bottom  in  a  flange  B,  to  which 
the  several  parts  forming  the  cistern  are  attached. 

The  upper  part  of  this  tube  is  cut  through  so  as  to  expose  the  glass 
tube  and  mercurial  column  within,  as  seen  in  figure.  Attached  at 
one  side  of  this  opening  is  a  scale,  graduated  in  inches  and  parts; 
and  inside  this  slides  a  short  tube  C,  connected  to  a  rackwork 
arrangement,  moved  by  a  milled  head  D;  this  sliding  tube  carries 
a  vernier  in  contact  with  the  scale,  which  reads  off  to  ^  (.002)  of 
an  inch. 

In  the  middle  of  the  brass  tube  is  fixed  the  thermometer  E,  the 
bulb  of  which  being  externally  covered,  but  inwardly  open,  and 
nearly  in  contact  with  the  glass  tube,  indicates  the  temperature  of 
the  mercury  in  the  barometer  tube,  not  that  of  the  external  air.  This 
central  position  of  the  thermometer  is  selected  that  the  mean  tem- 
perature of  the  whole  column  may  be  obtained,  a  matter  of  im- 
portance, as  the  temperature  of  the  barometric  column  must  be  taken 
into  account  in  every  scientific  application  of  its  observed  height- 

The  cistern  (Fig.  10)  is  made  up  of  a  glass  cylinder  F,  which  allows 
the  surface  of  the  mercury  q  to  be  seen,  and  a  top  plate  G,  through 
the  neck  of  which  the  barometer  tube  /  passes,  and  to  which  it  is 
fastened  by  a  piece  of  kid  leather,  making  a  strong  but  flexible 
joint.  To  this  plate,  also,  is  attached  a  small  ivory  point  //,  the 
extremity  of  which  marks  the  commencement  or  zero  of  the  scale 
above.  The  lower  part,  containing  the  mercury,  in  which  the  end 
of  the  barometer-tube  /  is  plunged,  is  formed  of  two  parts  /  /,  held 
together  by  four  screws  and  two  divided  rings  / ;;/,  in  the  manner 
shown  in  figure.  To  the  lower  piece  j  is  fastened  the  flexible  bag 
N,  made  of  kid  leather,  furnished  in  the  middle  with  a  socket  /£, 
which  rests  on  the  end  of  the  adjusting  screw  O.  These  parts,  with 
the  glass  cylinder  F,  are  clamped  to  the  flange  B  by  means  of  four 
long  screws  P  and  the  ring  R;  on  the  ring  R  screws  the  cap  S,  which 
covers  the  lower  parts  of  the  cistern,  and  supports  at  the  end  the 
adjusting  screw  O.  G,  i,j,  and  k  are  of  boxwood;  the  other  parts  are 
of  brass  or  German  silver.  The  screw  O  serves  to  adjust  the  mercury 
to  the  ivory  point,  and  also,  by  raising  the  bag,  so  as  to  completely 
fill  the  cistern  and  tube  with  mercury,  to  put  the  instrument  in  con- 
dition for  transportation. 

Some  cisterns  are  made  of  an  iron  or  glass  cylinder  with  a 
plunger  working  in  it  air-tight. 


37 


FIG.  9. 


FIG.  10. 


38 

CORRECTION    OF    BAROMETRIC    OBSERVATIONS. 

Corrections  must  be  applied  to  all  barometer  readings  in  order 
to  bring  the  indications  of  different  instruments  into  harmony  with 
each  other  before  they  can  be  used  for  comparative  purposes.  Some  of 
these  corrections  have  reference  to  the  special  instrument,  while 
others  are  applied  to  the  reading  of  any  instrument  taken  under  the 
same  conditions.  The  corrections  of  the  former  class  are  two  in 
number: 

I.  Instrumental  error. 
II.  Capillarity. 

Those  of  the  latter  class  are  also  two: 
•III.  Temperature. 
IV.  Altitude  above  sea  level. 

I.  Correction  for  instrumental  error. — This   is  applied   according  to 
the  error  discovered  in  the  individual  instrument  when  compared 
with  the  standard.     It  is  either  additive  (  +  )  if  the  barometer  reads 
too  low,  or  subtractive  ( — )  if  it  reads  too  high,  in  order  to  bring  it 
into  agreement  with  standard. 

II.  Correction  for  capillarity. — The  indications  of    barometers  are 
affected  by  the  capillary  action  between  the  glass  tube  and  the  mer- 
cury, the  effect  of  which  is  constantly  to  depress  the  mercury  by  a 
certain  quantity  nearly  inversely  proportional  to  the  diameter  of 
the  tube.     This  correction  is  included  in  the  correction  for  instru- 
mental error. 

The  correction  is  always  additive. 

This  depression  is  greater  in  tubes  in  which  the  mercury  has  not 
been  boiled  than  in  those  which  have  been  subjected  to  this  process. 

The  certificates  furnished  from  the  Weather  Bureau  Office  for  all 
barometers  verified  there  give  the  results  of  direct  readings  of  the 
columns  at  different  heights,  and  so  include  the  corrections  above 
mentioned,  in  so  far  as  any  of  them  are  applicable  to  the  special 
barometer  under  consideration. 

Correction  for  temperature,  or  reduction  (032°  F. — All  bodies  are  affected 
in  their  dimensions  by  heat;  with  few  exceptions  they  expand  when 
their  temperature  rises  and  contract  when  it  falls,  and  it  is  there- 
fore necessary,  in  taking  any  accurate  measure  of  pressure,  to  know 
at  what  temperature  it  was  taken,  in  order  that  we  may  know  what 
the  length  of  the  column  of  mercury  would  be  at  some  definite 
temperature,  which  is  taken  as  the  standard.  In  case  of  barometers 
this  standard  temperature  is  taken  as  32°,  and  accordingly  when 
the  barometer  is  at  a  temperature  below  32°,  considering  only  the 
mercurial  column,  the  correction  is  additive  (  +  ),  and  when  it  is 
above  32°  it  is  subtractive  ( — ). 

If  the  standard  temperature  for  the  brass  scale  were  the  same  as 
that  used  for  mercury,  the  sign  of  this  correction  would  change  from 


39 

+  to  —  at  32°,  but  the  standard  temperature  for  the  scale  is  regarded 
as  62°;  hence  the  sign  changes  at  29°  instead  of  32°,  the  total  cor- 
rection being  the  difference  of  the  two  corrections,  one  for  the 
temperature  of  the  mercury  and  the  other  for  the  temperature  of 
the  scale.  The  expansion  of  the  mercury  is  about  ten  times  as  great 
as  that  of  the  scale. 

The  temperature  of  the  barometer  is  given  by  the  attached  ther- 
mometer, the  bulb  of  which  is  so  placed  as  to  give  as  accurately  as 
possible  the  true  temperature  of  the  actual  column  of  mercury. 

The  corrections  are  given  in  the  table  for  each  half  inch  from  24  to 
31  inches,  as  of  course  the  correction  depends  on  the  length  of  the 
column  of  mercury. 

In  consequence  of  the  great  risk  of  the  heat  of  the  observer's  person 
affecting  the  thermometer  attached  to  the  instrument  during  the 
process  of  taking  a  reading  of  the  barometer,  the  attached  thermom- 
eter is  always  to  be  read  first  of  all,  before  the  reading  of  the  baro- 
metrical column  is  made. 

The  table  at  the  end  of  these  instructions  contains  the  corrections 
to  be  applied  to  the  readings  of  barometers  mounted  in  brass  frames, 
in  order  to  reduce  them  to  the  normal  temperature,  32°.  It  has  been 
computed  from  the  following  formula: 

Tm(*-32)  —  s«-62) 
Corrects  -ft-^-^-i^- 

ill  which 

7t=reading  of  the  barometer, 

<=temperature  of  attached  thermometer, 

w=rexpansion  of  mercury  for  1°  F.,  taken  as  .0001010  of  its  length  at  32°, 

s=expansion  of  the  substance  of  which  the  scale  is  made ;  for  brass  «  is  taken 

as  .00001020  of  its  length  (/t)  at  the  standard  temperature  for  the  scale* 

viz:  62°  F. 

CORRECTION    FOR     ALTITUDE    OR    REDUCTION    TO    SEA    LEVEL. 

As  we  ascend  in  the  atmosphere  the  pressure  gradually  diminishes- 
At  the  height  of  Pike's  Peak,  14,134  feet  above  sea  level,  the  mean 
pressure  is  about  17.8  inches.  At  sea  level  it  is  30.0  inches.*  In 
order  then  to  make  barometer  readings  comparable  at  different 
stations  it  is  necessary  to  reduce  them  to  a  common  plane.  This 
is  taken  as  sea  level.  This  reduction  depends  upon  the  temper- 
ature of  the  outside  air  as  well  as  the  height  of  the  station.  It  is 
important  that  each  observer  obtain  as  accurately  as  possible 
the  elevation  of  his  barometer,  first,  above  a  fixed  point  near  its  place 

*A  convenient  rule  for  finding  the  difference  of  level  between  two  places  by  means 
of  barometer  observations  is  as  follows:  The  difference  of  level  in  feet  is  equal  to  the 
difference  in  pressures  at  the  two  places  divided  by  the  sum  of  the  pressures  and  mul- 
tiplied by  the  number  55761  for  a  mean  temperature  of  the  air  at  the  places  equal  to 
60° ;  for  every  degree  the  temperature  is  greater  than  60°  the  number  is  117  greater, 
and  for  every  degree  below  60°  it  is  117  less.  For  example,  with  the  observed  pres- 
sure of  29.0  inches  at  a  height  and  30.0  inches  below  with  the  air  temperature  60°, 
the  difference  of  level  between  the  two  places  is,  ^  X  55761  =  945  feet. 


40 

of  suspension  ;  second,  the  elevation  of  the  fixed  point  above  some 
plane,  such  as  a  railroad  track  at  station,  bench-mark  on  canal,  or 
zero  of  river  gauge,  the  height  of  which  above  sea  level  is  known. 
The  sum  of  these  three  quantities  will  give  the  elevation  of  the 
barometer  above  sea  level.  If  it  is  found  impossible  to  get  the 
correct  elevation  in  this  way,  an  approximate  value  may  be  com- 
puted by  means  of  barometric  observations  at  the  station,  com- 
pared with  those  made  at  the  same  time  at  a  neighboring  station, 
the  elevation  of  which  is  known.  As  a  general  thing,  however, 
it  will  be  found  feasible  to  obtain  the  elevation  from  a  railroad  track 
near  the  station.  Table  VII  at  the  end  of  these  instructions  will 
give  the  reduction  for  barometer  readings  at  stations  up  to  1,500 
feet.  For  greater  heights  the  pressure  at  sea  level  may  be  com- 
puted from  the  following  formula: 


From  a  table  of  common  logarithms,  the  natural  number  corresponding  to  log  lf  is 
found;  OT  =n,  and  h=nli/ 

In  this  formula  — 
h  and  /j/=barometer  reduced  to  32°  F.,  at  sea  level  and  upper  station,  respect- 

ively, 

t  and  </=the  temperature  of  the  air  at  the  respective  stations, 
/^elevation  of  upper  station  in  feet, 
Platitude  of  the  place. 

Barometers  are  standardized  by  suspending  them  near  a  correct 
standard  barometer  and  shifting  the  scale  until  its  reading  coincides 
exactly  with  the  standard  or  its  correction  is  derived  to  reduce  to 
the  standard.  The  instrument  is  hung  with  lower  end  free.  If  the 
lower  end  is  fixed  for  convenience  in  adjusting  the  mercury  in  cis- 
tern to  the  ivory  point,  care  must  be  taken  to  fasten  it  without 
changing  the  verticality  of  the  tube.  Any  changes  in  the  suspend- 
ing ring,  putting  on  a  heavier  attached  thermometer,  or  shifting  the 
ivory  point  in  cistern,  is  apt  to  throw  the  instrument  out  of  adjust- 
ment and  cause  its  readings  to  be  erroneous  by  several  hundredths 
of  an  inch. 

To  make  pressure  observations  at  different  latitudes  strictly  com- 
parable, a  correction  must  be  introduced  for  the  variation  in  the 
force  of  gravity  on  the  different  parallels  from  the  equator  to  the 
poles.  The  reduction  is  made  to  the  latitude  of  45°  and  sea  level- 
At  latitude  30°  the  correction  amounts  to  —  0.039  °f  an  inch;  at  70° 
to  -j-  0.060. 

Pressures  can  be  observed  accurately  with  a  mercurial  barometer 
of  one-quarter  inch  bore  to  about  ±  0.006  of  an  inch.  For  more 
accurate  pressures  a  tube  a  half  inch  or  inch  in  diameter  is  required 
with  a  very  elaborate  apparatus  called  a  cathetometer,  for  measur- 
ing the  difference  in  level  of  the  surfaces  of  mercury  in  cistern  and 


41 

ANEROID     BAROMETER. 

Fig.  ii  represents  the  latest  improved  form  of  an  aneroid.  The 
outer  casing  and  face  of  the  instrument  are  removed,  but  the 
index  hand  is  left  attached  to  the  arbor.  A  is  the  corrugated 
vacuum  box  which  has  been  exhausted  of  air  through  the  tube  / 
and  hermetically  sealed  by  soldering.  £  is  a  powerful  curved  spring 
•resting  in  gudgeons  fixed  on  the  base  plate,  and  attached  to  a 

socket  behind  F,  in  the  top  of  the 
vacuum  box.  A  lever,  C,  joined  to 
the  stout  edge  of  the  spring,  is  con- 
nected by  the  bent  lever  at  Z>,  with 
the  chain  £,  the  other  end  of  which 
is  coiled  round  and  fastened  to  the 
arbor  F.  As  the  box  A  is  compressed 
by  the  weight  of  the  atmosphere 
increasing,  the  spring^,  is  tightened, 
the  lever  C  depressed,  and  the  chain 
FIG.  11.  E  uncoiled  from  F  which  is  thereby 

turned  so  that  the  hand  If  moves  to  the  right.  In  the  meanwhile 
the  spiral  spring  G  coiled  round  F,  and  fixed  at  one  extremity  to  the 
frame  work,  and  by  the  other  to  F,  is  compressed.  When,  therefore, 
the  pressure  decreases,  A  and  B  relax  by  virtue  of  their  elasticity; 
E  slackens,  G  unwinds,  turning  F,  which  carries  the  index  hand  If 
to  the  left.  Near  J  is  shown  an  iron  pillar,  cast  as  part  of  the  stock 
of  the  spring^.  A  screw  works  in  this  pillar  through  the  bottom  of 
the  plate,  by  means  of  which  the  spring  B  may  be  so  adjusted  to  the 
box  A  as  to  set  the  index  H  to  read  on  the  scale  in  accordance  with 
the  indications  of  a  mercurial  barometer.  In  the  higher  class  of 
aneroid  barometers  the  lever  Cis  formed  of  a  compound  bar  of  brass 
and  steel,  so  arranged  as  to  compensate  for  the  effects  of  extreme  vari- 
ations of  temperature.  A  very  small  movement  of  the  surface  of  the 
vacuum  chamber  causes  a  large  deviation  of  the  needle  end,  ¥£Q  of 
an  inch  causing  it  to  move  through  a  space  of  3  inches. 

Aneroids  have  to  be  graduated  with  reference  to  a  mercurial 
barometer. 

The  elasticity  of  a  spring  varies  with  temperature.  A  given  weight; 
will  pull  down  a  spring  more  at  a  high  temperature  than  a  low  one. 
This  causes  an  aneroid  to  read  too  high  with  temperature.  In  the 
best  forms  of  aneroid  there  is  an  attempt  made  to  compensate  for 
temperature,  by  making  one  of  the  transmitting  levers  a  compound 
of  two  metals,  a  strip  of  steel  and  brass  soldered  lengthwise.  An 
accurate  compensation  is  only  rarely  accomplished  in  this  way,  and 
when  it  is  accomplished,  in  the  case  of  some  instruments,  it  is  a  mere 
accident.  To  get  correct  pressures  every  instrument  must  be 
investigated  and  corrections  for  different  pressures  and  temper- 
atures derived.  A  change  of  20°  in  the  temperature  of  the 


42 

instrument  will  often  change  the  pressure  indication  one-tenth  of 
an  inch,  in  some  instruments  one  way  and  in  others  the  opposite, 
without  any  actual  change  going  on  in  the  pressure  of  the  air. 

A  compensation  is  sometimes  attempted  by  leaving  some  air  in 
the  box.  As  the  spring  weakens  at  a  high  temperature  the  increas- 
ing pressure  of  the  air  inside  the  box,  due  to  the  higher  temperature, 
counterbalances  the  effect  on  the  spring  to  some  extent.  This 
method  of  compensation  is  usually  ineffective. 

Aneroid  barometers  do  not  require  a  gravity  correction,  the  ten- 
sion of  a  spring  not  varying  with  latitude. 

The  spring  of  an  aneroid  barometer  changes  with  age  and  with 
extreme  variations  of  pressure  causing  its  indications  to  vary  inde- 
pendently of  variations  in  the  pressure,  of  the  atmosphere.  There 
is  a  continual  rise  in  the  indicated  pressures  with  age  of  the  instru- 
ment, amounting,  usually,  to  0.08  of  an  inch  in  three  years.  After 
subjection  to  a  low  pressure,  as  in  a  mountain  ascent,  an  aneroid 
does  not  recover  its  original  reading  at  once  when  restored  to 
ordinary  pressures.  After  lowering  to  a  pressure  of  18  inches  its 
reading  will  be  0.3  of  an  inch  lower  than  it  was  before  when  brought 
back  to  a  pressure  of  30  inches.  The  original  reading  is  recovered 
in  a  few  weeks. 

An  aneroid  requires  constant  control  and  adjustment  by  reference 
to  a  mercurial  barometer.  It  is  not  as  accurate  or  satisfactory  for 
making  pressure  observations  as  a  mercurial  instrument.  Shocks 
and  jars  are  liable  to  derange  it.  On  account,  however,  of  its  conve- 
nience, the  ease  with  which  it  can  be  carried  over  rough  country 
where  a  mercurial  barometer  would  be  liable  to  be  broken,  the 
aneroid  is  used  a  good  deal  in  making  observations  of  pressure, 
especially  for  approximate  determinations  of  altitude.  An  aneroid 
is  rarely  used  in  meteorological  observations,  usually  only  when  a 
mercurial  barometer  is  out  of  order  and  until  another  one  can  be 
procured. 

The  words  "fair,"  "stormy,"  "rain,"  etc.,  sometimes  on  the  dial 
of  an  aneroid  have  no  real  significance  as  regards  weather,  being 
simply  a  device  intended  to  help  the  sale  of  the  instruments. 

HYPSOMETER. 

The  temperature  of  boiling  water  is  lower,  the  lower  the  pressure 
of  the  air.  The  boiling  point  at  a  standard  pressure  of  29.922  inches 
is  2i2°.o;  at  a  pressure  of  18.0  inches  it  is  i870.5.  This  fact  is  made 
use  of  at  times  in  determining  pressures,  especially  for  the  deter- 
mination of  altitudes  in  mountain  ascents. 

A  thermometer  with  its  boiling  point  apparatus  when  used  for  the 
purpose  of  determining  pressures  is  called  a  hypsometer.  The  ther- 
mometer is  made  short,  only  about  6  inches  in  length,  for  convenience 
in  carrying,  and  is  graduated  only  from  about  170°  to  214°.  The 
best  form  of  thermometer  for  this  purpose  has  two  or  three  degrees 


43 

in  the  vicinity  of  freezing  point  graduated  on  the  stem  to  permit  of 
observations  of  the  variation  of  the  freezing  point.  An  inconvenient 
length  of  stem  is  avoided  by  means  of  an  enlargement  in  the  bore 
of  the  stem,  between  freezing  point  and  the  graduation  of  170°,  in 
order  to  hold  the  mercury. 

The  pressure  observed  with  a  hypsometer  can  be  made  with  an 
accuracy  of  about  0.06  of  an  inch. 

READING    INSTRUMENTS. 

In  reading  thermometers  or  barometers,  it  is  essential  to  bring  the 
eye  so  that  the  line  from  the  eye  to  the  end  of  the  column  shall  be 
horizontal.  This  is  most  important  in  case  the  thermometer  is 
graduated  only  on  the  brass  scale  behind  the  tube,  as  a  little  varia- 
tion in  the  position  of  the  eye  above  or  below  the  line  of  graduation 
will  make  a  large  difference  in  the  reading.  Care  should  be  taken 
in  the  reading  to  note  correctly  the  number  of  degrees  that  the 
thermometer  indicates.  It  has  been  found  that  frequently  errors  of 
5°  creep  in  from  a  want  of  care  in  this  respect.  A  little  practice  will 
enable  any  one  to  split  the  degree  into  tenths  ;  /.  e.t  to  regard  the 
space  as  divided  into  ten  equal  parts  and  to  take  the  nearest  tenth 
that  the  instrument  indicates  for  record. 

RAIN  GAUGE. 

Rainfall  is  measured  as  the  depth  of  water  it  would  form  on  the 
ground  were  it  to  remain  as  it  falls.  In  the  case  of  snow  its  depth 
is  measured  as  the  depth  of  water  it  would  form  if  melted.  Rain, 
hail,  and  snow  are  known  under  the  general  designation  of  precipi- 
tation. 

Snowfall  is  preferably  measured  as  depth  of  water  rather  than  by 
the  thickness  of  layer  it  forms  on  the  ground.  When  it  is  not  con- 
venient to  measure  it  by  melting,  it  is  customary  to  take  it  as  one- 
tenth  of  the  measured  depth  of  the  snow  on  a  level  open  place.  The 
relation  between  the  depth  of  snow  and  depth  of  melted  snow  is 
very  different  in  different  cases,  depending  on  the  wetness  of  the 
snow.  The  equivalent  depth  of  water  in  some  cases  is  as  great  as 
one-seventh  of  the  depth  of  snow  and  in  others  only  one  thirty- 
fourth.  It  is  always  best  to  melt  it  for  measurement. 

Rain-gauges  in  slightly  different  positions  differ  greatly  in  the 
depth  of  rain  indicated.  Within  a  few  yards  of  each  other  two 
gauges  may  show  a  difference  of  20  per  cent,  in  the  rainfall  in  a 
heavy  rainstorm.  The  stronger  the  wind  the  greater  the  difference 
is  apt  to  be.  In  a  high  location  eddies  of  wind  produced  by  walls 
of  buildings  divert  rain  that  would  otherwise  fall  in  the  gauge.  A 
gauge  near  the  edge  of  the  roof,  on  the  windward  side  of  a  building, 
shows  a  less  rainfall  than  one  in  the  center  of  the  roof.  The  verti- 
cal ascending  current  along  the  side  of  the  wall  extends  slightly 
above  the  level  of  the  roof,  and  part  of  the  rain  is  carried  away  from 
the  gauge. 


44 

In  the  center  of  a  large  flat  roof,  at  least  60  feet  square,  the  rain- 
fall collected  by  a  gauge  does  not  differ  materially  from  what  is 
collected  at  the  level  of  the  ground. 

A  rain-gauge  mounted  on  a  mast  at  a  height  of  43  feet  collects 
only  0.75  as  much  as  at  the  ground ;  at  85  feet,  only  0.64 ;  and  at  194 
feet,  only  0.58. 

A  gauge  on  a  plain  with  a  fence  3  feet  high  around  it  at  a  distance 
of  3  feet  will  collect  6  per  cent,  more  rain  than  without  the  fence. 
These  differences  are  due  entirely  to  wind  currents. 

Forests  intercept  from  6  to  15  per  cent,  of  the  rainfall  in  different 
rain  storms. 

INSTRUCTIONS    FOR    USE    OF    THE    RAIN-GAUGE. 

The  Weather  Bureau  solicits  the  observation  and  record  of  rain, 
snow,  and  hail,  at  carefully  selected  stations,  with  a  three-fold  object: 
to  benefit  commerce  on  navigable  streams,  to  assist  the  operations 
of  agriculture  by  irrigation  and  water  storage  in  regions  where  the 
rainfall  is  insufficient  at  the  time  when  the  crops  most  require  water, 
to  enable  specialists  of  the  bureau  to  gain  such  knowledge  of  the 
climate  of  the  several  regions  of  the  country  as  will  permit  them  to 
supply  engineers  and  others  with  the  facts  which  are  needed  to  govern 
them  in  the  construction  of  waterworks  and  other  undertakings. 
These  eminently  practical  benefits  are  to  be  obtained  only  by  care- 
fully prepared  records  of  observations  of  the  rainfall  made  at  many 
stations.  To  secure  accuracy  each  observer  should  study  the  in- 
structions herewith  offered  for  his  guidance. 

Each  station  will  be  equipped  with  a  standard  rain-guage,  and,  if 
possible,  a  wooden  box  support,  complete,  which  is  to  be  mounted 
in  the  manner  described  below. 

The  rain-gauge  consists  of  three  parts — 
The  receiver,  A. 
The  overflow  attachment,  B. 
The  measuring  tube  C. 

The  top  cylindrical  portion  of  the  receiver,  marked  a  in  Fig.  12,  is 
exactly  8  inches  in  diameter  inside,  and  is  provided  with  a  funnel- 
shaped  bottom,  which  conducts  any  precipitation  falling  into  the 
receiver  into  the  tall  cylindrical  measuring  tube,  C,  the  total  height 
of  which,  inside,  is  exactly  20  inches.  The  diameter  of  this  tube  is 
much  smaller  than  the  large  receiving  tube,  a,  being  only  2.53  inches. 
In  consequence  of  this  a  small  amount  of  rain  falling  into  the  re- 
ceiver and  flowing  into  C  fills  the  latter  to  a  depth  greater  than  the 
actual  rainfall  in  proportion  as  the  area  of  the  receiver  is  greater 
than  the  area  of  the  measuring  tube.  In  the  standard  gauges  of  the 
Weather  Bureau  the  depth  of  the  rainfall,  in  accordance  with  this 
principle,  is  magnified  just  ten  times.  The  receiver,  A,  has  a  sleeve, 
</,  Fig.  13,  which  slips  over  the  tube,  C,  and  very  effectually  prevents 


45 


any  loss  of  rainfall.  Again,  when  the  rainfall  is  very  heavy  the 
tube,  C,  may  be  more  than  filled.  In  this  case  to  still  prevent  loss  a 
little  opening,  shown  at  <?,  Fig.  14,  is  made  in  the  sleeve,  d>  just  on  a 
level  with  the  top  of  the  tube,  C.  The  excess  of  rainfall  escapes 
through  this  opening,  and  is  retained  in  the  large  overflow  attach- 
ment, B,  and  can  be  measured  afterwards,  as  will  be  described  below. 
In  the  drawing  the  diameter  of  the  overflow  attachment  is  shown  as 
less  than  8  inches,  but  in  the  latest  style  gauges  this  part  is  now  also 
made  just  8  inches  inside  diameter.  The  object  of  this  is  to  be  able 
to  use  this  portion  of  the  instrument  as  a  snow  gauge,  as  will  be  ex- 
plained hereafter. 


Fig.lZ. 
Front  View. 


Pig.  13. 
Vertical  Section. 


RAIN  GAUGE. 


\ 

--. 

f 

4 

B 

? 

C 

d 
3 

Receiver. 


Fig.15. 
Horizontal  Section,E-i; 


0     I    234    56    7    Q    9    10   II    12   13  14  IS-  16  17  IB  19  20  21 


&  \*uu». 


SCALE. 
EXPOSURE  OF  RAIN-GAUGES. 

It  is  desired  observers  use  particular  care  in  selecting  a  good  place 
of  observation,  as  the  value  of  the  records  is  sometimes  greatly  im- 
paired by  improper  exposure.  It  is  scarcely  necessary  to  say  that 
every  precaution  should  be  taken  to  protect  gauges  from  the  inter- 
ference of  animals  and  unauthorized  persons.  Select,  if  possible,  a 
position  in  some  open  lot  as  unobstructed  as  possible  by  trees,  build- 
ings, or  fences.  Such  a  place  in  general  affords  the  best  exposure, 
though  sometimes  difficult  to  find.  Gauges  should  be  exposed  upon 
roofs  of  buildings  only  when  necessary,  and  then  the  roof  should  be 
flat,  or  nearly  so.  The  middle  portion  of  a  flat  unobstructed  roof 
generally  gives  the  best  results. 


46 


RAIN-GAUGE  SUPPORT. 

The  box  in  which  the  gauge  is  shipped  to  the  observer  is  expressly 
designed  as  a  stand  for  the  instrument,  and  should  be  opened  at  the 
head,  which  is  fastened  by  screws.  Set  the  box  up  as  nearly  vertical 
as  possible  at  the  place  selected  for  the  exposure  and  ballast  by  fill- 
ing the  lower  portion  with  several  inches  of  stone  or  broken  brick. 
Slip  in  the  head  and  lower  to  the  level  of  the  screw  holes  in  the  sides 
of  the  box  about  10  inches  from  the  bottom,  where  the  head  is 
securely  fastened  with  the  screws  taken  out  in  opening  the  box. 
The  support  is  further  secured  and  fixed  in  its  position  by  piling 
up  a  few  stones  around  the  outside.  The  gauge  can  now  be  placed 
inside  and  appears  as  shown  in  Fig.  16. 

Ratn  Gauge  cutoi  Support. 


FIG.  16. 

TO  MEASURE  RAINFALL  AND  SNOWFALL. 

Rainfall. — The  rain-gauge  measuring  stick  is  graduated  into  inches 
and  tenths  of  inches.  Remembering  that  the  actual  depth  of  the 
rainfall  is  magnified  ten  times,  as  explained  above,  it  is  plain  that 
if  we  find  the  water  10  inches  deep  .in  the  measuring  tube,  then  the 
real  rainfall  must  have  been  only  one  inch  deep,  or,  if  the  water  in 
the  tube  is  only  one-tenth  inch  (or  written  as  a  decimal  .1  inch)  deep, 
then  the  rainfall  must  have  been  only  one  one-hundreth  inch  (or 
written  as  a  decimal  .01  inch). 


47 

To  save  observers  the  trouble  of  always  thinking  about  the  mag- 
nification, and  to  avoid  possible  errors  in  reports,  the  numbers  on 
the  graduation  of  the  measuring  sticks  are  not  actual  inches,  but  in 
the  latest  pattern  of  measuring  sticks  have  all  been  divided  by  10, 
and  thus  represent  the  actual  rainfall.  Moreover,  these  numbers 
are  expressed  in  hundredths  of  inches  of  rainfall,  and  are  written 
as  decimal  fractions.  Thus  the  ten-inch  lige  is  numbered  i.oo  (read 
one  and  zero  hundreths),  which  is  the  depth  of  rainfall  in  inches 
corresponding  to  i-o  inches  of  water  in  the  measuring  tube  ;  similarly 
the  one-inch  line  is  numbered  .10  (read  ten  one-hundredths),  which 
again  is  the  depth  of  rainfall  in  inches  corresponding  to  one  inch  of 
water  in  the  tube. 

The  depth  of  the  water  is  measured  by  inserting  the  measuring 
stick  into  the  gauge  through  the  small  hole  in  the  funnel.  When 
the  stick  reaches  the  bottom  of  the  measuring  tube  it  should  be  held 
for  one  or  two  seconds  and  then  quickly  withdrawn  and  examined 
to  see  at  what  division  of  the  graduation  the  top  of  the  wetted  por- 
tion comes.  The  numbering  of  this  division  as  stamped  on  the  stick 
gives,  as  has  just  been  explained,  the  actual  depth  of  rainfall,  and  in 
making  out  records  and  reports  observers  should  always  use  the  decimal 
expressions.  Of  course  it  will  rarely  happen  that  the  top  of  the 
wetted  portion  will  fall  exactly  upon  one  of  the  numbered  lines — it 
will  generally  be  on  or  near  one  of  the  shortest  lines.  Thus,  for 
example,  suppose  the  water-mark  comes  to  the  sixth  short  line 
beyond  the  line  numbered  .80,  the  proper  record  to  make  in  this  case 
would  be  .86  inch  rainfall.  The  number  of  short  lines,  reckoned  from 
the  numbered  line  next  lower,  are  always  to  be  inserted  in  place  of 
the  o  in  the  stamped  numbers. 

Observers  should  always  be  careful  to  put  the  stick  into  the  gauge 
so  that  the  end  at  which  the  numbering  begins  goes  to  the  bottom, 
and  the  stick  passes  through  the  middle  of  the  tube;  for  if  the  stick 
is  placed  near  the  sides  the  water  is  sometimes  drawn  up  by  capil- 
lary action  in  the  narrow  space  between  the  stick  and  the  tube,  so  as 
to  wet  the  former  entirely  too  high  and  give  very  erroneous  records. 

After  measuring  and  recording  in  this  way  the  water  found  in 
the  gauge  the  top  should  be  removed,  the  measuring  tube  emptied 
and  drained,  and  the  gauge  put  in  position  again.  Observers  should 
be  careful  after  emptying  the  gauge  to  replace  the  measuring  tube 
so  that  the  bottom  stands  within  the  ring  in  the  middle  of  the 
bottom  of  the  overflow,  and  in  putting  on  the  receiver  that  it  passes 
over  the  measuring  tube  and  rests  squarely  down  upon  the  overflow. 

When  the  amount  of  rain  that  has  fallen  more  than  fills  the  meas- 
uring tube,  the  total  rainfall  is  determined  in  the  following  manner: 
First  carefully  remove  the  receiver  so  as  not  to  spill  any  of  the 
water  in  the  measuring  tube,  which  should  be  exactly  full;  the  latter 
is  then  slowly  and  carefully  lifted  out  so  as  not  to  spill  any  of  the 


48 

water  into  the  overflow,  emptied  and  allowed  to  drain  a  moment  or 
so;  the  water  remaining  in  the  overflow  is  now  poured  into  the 
measuring  tube,  being  careful  not  to  lose  any,  and  measured  in  the 
usual  way.  Suppose  we  find  this  to  be  .47  inch  rainfall,  then,  re- 
membering that  the  measuring  tube  is  just  20  inches  high,  the  total 
rainfall  will  be  2  inches  +  .47  inch=2.47  inches. 

Snowfall. — During  the  winter  season,  especially  in  those  climates 
where  the  precipitation^  nearly  all  in  the  form  of  snow,  it  is  neces- 
sary to  have  only  the  overflow  attachment  exposed  in  the  support 
as  a  snow-gauge,  removing  the  receiver  and  measuring  tube  to  the 
house,  as  these  parts  cannot  be  used  for  measuring  snow,  and  even 
if  rain  should  occur  it  is  very  apt  to  be  frozen  while  in  the  measur- 
ing tube,  generally  bursting  it  and  rendering  it  worthless  or  highly 
inaccurate. 

The  snowfall  collected  in  the  overflow  attachment  is  measured  by 
first  placing  the  vessel  in  a  warm  room  until  the  snow  is  melted. 
The  water  is  then  carefully  poured  into  the  measuring  tube  and 
measured  just  as  though  it  were  rainfall. 

In  addition  to  this  measurement  by  the  gauge  a  measurement  will 
be  made  of  the  actual  depth  in  inches  of  the  snow  on  the  ground. 
Select  a  level  place  of  some  extent  where  the  drifting  is  least  pro- 
nounced and  measure  the  snow  in  at  least  three  places.  The  mean 
of  these  measurements  will  give  the  snowfall  which  is  to  be  entered 
in  the  fourth  column  of  the  report,  and  whenever  it  is  impracticable 
to  melt  the  snow  as  described  in  the  preceding  paragraph,  one-tenth 
of  this  mean  will  give  an  approximate  value,  in  water,  for  the  snow 
which  could  not  be  melted.  This  value  must  be  set  down  in  the 
third  column  of  the  report  in  precisely  the  same  manner  as  rainfall, 
or  snow  melted  in  the  gauge.  After  having  once  made  a  measure- 
ment of  the  snowfall  it  is  not  desired  that  the  same  snow  be  meas- 
ured at  each  succeeding  observation  until  it  shall  finally  disappear. 
Any  fresh  snow,  however,  should  be  measured  and  recorded  as  it 
falls.  If  there  be  any  snow  on  the  ground  at  the  middle  and  end  of 
the  month  that  fact  and  the  depth  in  inches  should  be  noted. 

Observations  should  be  made  every  evening,  also  at  the  close  of 
every  storm,  and  the  gauge  should  be  emptied  of  all  the  water  it  may 
contain  as  soon  as  it  has  been  measured. 

The  form  supplied  for  these  records  will  be  found  self-explanatory. 
Rainfall  observers  will  disregard  the  columns  headed  "Tempera- 
ture," and  confine  their  attention  to  those  headed  "  Precipitation." 
The  first  and  second  columns  have  to  do  with  the  time  of  beginning 
and  ending  of  the  storm,  and  together  measure  its  duration.  In 
the  third  column  is  to  be  entered  the  reading  of  the  gauge,  as  de- 
scribed on  pages  46  and  47  of  these  instructions.  If  no  rain, 
snow,  or  hail  has  fallen  during  the  period  of  observation  make  the 
entry  .00,  or  "No  precipitation,"  in  this  column.  If  the  amount  is 


49 

too  small  to  measure  make  the  entry  "  Trace."  The  fourth  column 
is  provided  for  the  record  of  snow  measurements  made  by  the 
method  described  on  page  48.  In  the  "Monthly  Summary"  all 
matters  indicated  are  proper  subjects  of  the  rainfall  observer's 
record,  with  the  exception  of  the  three  lines  which  deal  with  tem- 
perature. 

It  is  particularly  important  in  the  interests  of  accuracy  that  the 
observations  be  recorded  as  soon  as  made,  and  that  the  daily  entries 
be  made  day  by  day.  The  forms  for  each  month  should  be  mailed 
on  the  first  day  of  the  succeeding  month.  Even  if  no  rain  has  fallen 
the  observer  should  bear  in  mind  that  his  official  record  of  that  fact 
is  as  important  as  though  it  were  a  month  of  rain. 

Requisition  for  blank  forms  or  envelopes  may  be  made  at  any 
time,  and  will  receive  immediate  attention.  It  is  not  necessary  to 
write  a  letter  asking  for  them;  a  simple  note  on  the  margin  of  the 
monthly  report  will  be  sufficient. 

PERCOLATION  GAUGE. 

An  instrument  for  measuring  the  rainfall  that  reaches  different 
depths  in  the  earth  is  a  percolation  gauge,  sometimes  called  a 
lysimeter.  It  consists  of  an  iron  vessel,  imbedded  in  the  earth, 
three  feet  in  diameter  and  three  feet  deep,  filled  with  earth,  and  the 
top  surface  level  with  the  ground.  There  is  a  pump  for  drawing 
off  the  water  that  collects  in  a  depression  on  one  side  at  the  bottom 
of  the  vessel,  which  then  has  to  be  measured. 

The  amount  of  rainfall  that  reaches  a  depth  of  three  feet  is  only 
about  one-third  or  one-fourth  of  the  annual  depth  of  rainfall  as 
measured  at  the  surface  of  the  ground.  It  varies  a  good  deal  with 
the  nature  of  the  soil. 

WIND  VANE. 

The  wind  vane  is  the  instrument  by  which  the  direction  of  the 
wind  is  observed.  The  direction  of  the  wind  is  always  considered 
as  the  direction  from  which  it  is  coming.  The  direction  is  noted  to 
the  nearest  principal  point  of  the  compass  from  which  it  is  blowing, 
north,  northeast,  east,  southeast,  south,  southwest,  west,  or  northwest. 

A  calm  is  recorded  when  there  is  not  sufficient  wind  to  move  the 
anemometer  cups. 

The  motion  of  smoke  from  chimneys  can  be  used  to  observe  the 
direction  of  feeble  currents  of  air,  too  weak  to  turn  the  wind  vane. 

The  Weather  Bureau  form  of  wind  vane  is  shown  in  Fig.  17.  The 
arrow  is  about  6  feet  long ;  the  tail  piece  is  of  pine  10  inches  wide, 
split  to  form  an  angle  'of  about  10°.  This  is  considered  to 
steady  the  direction  of  the  vane  and  render  it  more  sensitive  to  the 
action  of  light  currents  than  if  it  were  a  plain  board. 

The  wind  vane  should  be  set  in  as  free  and  open  a  space  as 
possible,  away  from  every  obstacle,  especially  high  buildings.  It 
should  be  8  or  10  feet  above  a  roof. 


Friction.  Roller. 


Wind  Vcvne  and,  Ceittng  Died. 
FIG.  17. 


51 

In  the  complete  form  of  wind  vane  there  is  a  rod  connecting  with 
the  vane,  passing  through  the  support  and  the  roof  of  building  and 
attached  to  an  index  centered  in  a  dial  on  the  ceiling  of  a  room. 
The  principal  points  of  the  compass  are  marked  on  dial,  and  the 
wind  direction  can  be  noted  without  direct  observation  of  the  vane. 

In  noting  changes  in  the  direction  of  wind  the  terms  veering  and 
backing  are  used.  Veering  is  a  change  in  the  direction  from  the 
north  to  the  northeast,  through  the  east  to  the  southeast,  southeast 
to  south  and  southwest,  etc.,  corresponding  to  fhe  direction  of  mo- 
tion of  the  hands  of  a  watch.  Backing  is  changing  in  the  reverse 
direction,  around  by  north,  through  the  west,  etc. 

ANEMOMETER. 

The  instrument  generally  used  in  determining  the  velocity  of  the 
wind  is  the  Robinson  anemometer,  shown  in  Fig.  18. 


FIG.  18. 

It  consists  of  four  hollow  hemispherical  cups  upon  cross-arms  at 
right  angles  to  each  other,  with  the  open  sections  vertical  and  facing 
the  same  way  around  the  circumference. 

The  cross-arms  are  on  a  vertical  axis  which  has  at  its  lower  end 
an  endless  screw.  The  axis  is  supported  so  as  to  turn  with  as  little 
friction  as  possible.  The  endless  screw  is  in  gear  with  a  wheel 
which  moves  two  dials  registering  the  number  of  revolutions  of  the 
cups. 

9377 4 


52 

The  center  of  the  cups  moves  with  a  velocity  about  one-third 
that  of  the  wind  which  puts  them  in  motion. 

The  cups  are  4  inches  in  diameter.  The  distance  from  center  of 
cup  to  center  of  rotation  or  axis  is  6.72  inches.  On  the  assumption 
that  the  wind  travel  is  exactly  three  times  that  of  the  center  of  cup, 
the  dials  are  marked  to  register  miles  of  wind  travel,  500  revolu- 
tions of  the  cups  corresponding  to  a  mile. 

The  ratio  of  wind  travel  to  travel  of  cup  is  in  reality  variable, 
depending  on  the  velocity  of  the  wind.  It  is  less  for  high  than  low 
velocities.  It  varies  also  with  the  dimensions  of  the  instrument, 
being  different  for  every  different  length  of  arm  and  diameter  of  cup. 

The  true  velocities  corresponding  to  observed  velocities  areas  fol- 
lows : 

Observed  ve-  True  velocity, 

locity,  miles  miles  per 

per  hour.  hour. 

0 

10  9.8 

20  17. 8 

30  25. 7 

40  33. 3 

50  40. 8 

60  48. 0 

70  55. 2 

80  62. 2 

90  69. 2 

The  pattern  of  anemometer  used  in  the  Weather  Bureau  has  the 
registering  dials  mounted  concentrically.  The  outer  dial  has  one 
hundred  and  the  inner  dial  ninety-nine  divisions.  As  the  dials  are 
moved  by  the  same  wheel,  they  will  move  forward  one  hundred 
divisions  in  the  same  time  (Fig.  19).  The  outer  dial  having  one 
hundred  divisions,  the  inner  dial  will  complete  one  revolution  and 
its  zero  be  one  division  beyond  or  to  the  left  of  the  zero  of  the  outer 
dial  when  the  outer  dial  has  completed  one  revolution,  the  zeros  of 
the  scales  coinciding  at  the  time  the  instrument  was  set  in  motion. 
Thus  the  revolutions  made  by  the  outer  dial  are  recorded  on  the 
innei-  one,  the  number  of  revolutions  being  shown  by  the  number 
of  divisions  of  the  scale  on  the  inner  dial  between  the  zero  of  that 
scale  and  the  zero  of  the  outer  one.  In  taking  the  reading  of  the 
anemometer  at  any  time,  the  hundreds  and  tens  of  miles  are  read 
from  the  inner  scale  and  the  miles  and  tenths  of  miles  are  read  from 
the  outer  one.  Take  from  the  inner  scale  the  hundreds  and  tens  of 
miles  contained  between  the  zero  of  that  scale  and  the  zero  of  the 
outer  one,  and  the  miles  and  tens  of  miles  on  the  outer  scale  con- 
tained between  the  zero  of  that  scale  and  the  index  of  the  instru- 
ment, and  the  sum  of  these  readings  will  be  the  reading  of  the 
instrument  at  the  time  of  making  the  observation. 

When  the  anemometer  is  not  furnished  with  an  index  point  the 
center  of  the  small  wheel  which  gives  motion  to  the  dials  will  be 
taken  as  the  reference  point. 


53 


The  total  movement  for  the  twenty-four  hours  wil  be  obtained 
in  the  following  manner :  Subtract  the  reading  of  the  anemometer 
at  12  noon  of  the  preceding  day  from  the  reading  taken  at  12  noon 
of  the  current  day,  and  the  difference  will  be  the  total  movement  of 
the  wind.  When  the  reading  of  the  anemometer  is  less  than  the 
reading  of  the  preceding  day,  990  miles  will  be  added  to  it,  and  the 
remainder,  after  sulitracting  the  reading  of  the  preceding  day,  will 
be  the  total  movement. 

Example :  The*  dial  reading  of  to-day  is  91,  and  that  of  yesterday 
was  950,  hence  we  have  91  +  990  =  1081  ;  1081 — 950=131,  the  total 
observed  movement  of  the  wind  in  miles  during  the  past  twenty- 
four  hours. 

Anemometers  must  be  kept  carefully  and  thoroughly  oiled  to 
prevent  friction  and  injury  to  the  several  bearings.  Especial 
attention  must  be  given  to  the  large  dial-screw,  and  when  found 
loosened  it  must  at  once  be  tightened,  but  care  must  be  taken  not 
to  screw  it  up  tight  enough  to  interfere  with  the  free  motion  of  the 
dials. 

The  pressure  of  the  wind  on  a  surface  varies  as  the  square  of  the 
velocity.  The  pressure  in  pounds  per  square  foot  of  vertical  expos- 
ure of  a  surface  is  equal  to  0.004  multiplied  by  the  square  of  the  true 
wind  velocity  in  miles  per  hour. 

When  there  is  no  anemometer  for  measuring  the  velocity,  the 
force  of  wind  is  estimated  according  to  a  scale  of  numbers  from  i  to 
12.  The  scale  in  most  common  use,  especially  at  sea,  is  the  Beaufort 
scale.  Estimates  on  this  scale  at  sea  are  made  for  the  most  part 
according  to  the  commotion  the  wind  causes  in  the  water  or  the 
rigging  of  a  ship.  This  arbitrary  scale  originated  in  the  days  of 
sailing  vessels.  The  wind  force  was  indicated  by  the  various  num- 
bers according  to  the  amount  of  sail  that  could  be  safely  carried. 
The  velocity  in  miles  per  hour  corresponding  to  the  different  num- 
bers has  been  ascertained  by  comparison  of  the  estimated  force  on 
ships  at  sea  with  actual  velocities  observed  by  anemometers  on 
shore  in  the  vicinity  at  the  same  time. 

The  scale  is  as  follows: 


Wind  force, 
Beaufort  scale. 

Velocity, 
miles  per  hour. 

0 

Calm          

0 

1 

Light  air 

3 

2 

Light  breeze    _     

13 

3 

Gentle  breeze 

18 

4      _ 

Moderate  breeze    _  _ 

23 

5 

Fresh  breeze 

28 

6 

34 

7 

Moderate  gale           

40 

8 

48 

9     _ 

Strong  gale        

56 

10 

Whole  gale 

65 

11 

Storm 

75 

12    — 

Hurricane             _         _    „ 

90 

64 

The  above  velocities  are  not  corrected  for  the  errors  of  anemom- 
eter. The  velocities  are  those  observed  with  the  English  pattern 
of  anemometer  at  coast  stations  in  Great  Britain.  This  anemometer 
has  larger  cups  and  longer  arms  than  the  Weather  Bureau  pattern  of 
instrument.  It  is  geared  and  dialed  on  the  supposition  that  the 


FIG.  19. — ANEMOMETER  DIAL. 

wind  travel  is  exactly  three  times  the  travel  of  center  of  cups.  At 
30  miles  an  hour  it  is  probably  2.4  times  the  velocity  of  cup  instead 
of  3.0,  as  shown  by  Dohrandt's  experiments  with  specimens  of  the 
instrument  at  St.  Petersburgh,  which  makes  an  indicated  velocity 
of  30  miles  correspond  to  a  true  velocity  of  24. 


55 

CLOUDS. 

There  is  a  very  great  variety  of  cloud  forms.  Seven  typical  forms 
are  recognized  for  meteorological  observation  according  to  Howard's 
classification. 

Cirrus. 

Cumulus. 

Stratus. 

Cirro-cumulus. 

Cirro-stratus. 

Cumulo-stratus. 

Nimbus. 

Cirrus. — Cirrus  cloud,  the  highest  of  all  clouds,  is  a  streaky,  gauzy, 
wispy  or  feathery  form  of  cloud,  whitish  in  color,  usually  not  very 
abundant.  It  forms  at  great  heights,  25,000  to  50,000  feet,  rarely 
occurring  below  16,000  feet.  It  is  sometimes  known  as  "cat's  whisk- 
ers" and  "mares'  tails." 

Cumulus. — Cumulus  cloud  is  masses  of  compact  cloud  of  a  dense 
rounded  appearance,  like  cotton  bulging  from  a  bale.  It  has  a  flat, 
horizontal  base  3,000  to  5,000  feet  above  the  ground,  and  towers  up 
conically  to  a  great  height  in  the  sky.  Its  shape  indicates  it  is  the 
result  of  the  condensation  of  moisture  as  the  air  in  ascending  cools 
dynamically  to  the  dew  point.  Cumulus  cloud  forms  the  visible 
capital  of  an  ascending  column  of  air.  It  is  essentially  a  day  cloud. 
It  is  more  common  in  the  afternoon  than  the  morning,  and  is  most 
noticeable  when  the  sun  is  low.  It  is  frequent  at  all  times  of  the 
year. 

The  distant  tall  tops  of  cumulus  visible  on  the  horizon  at  sea  are 
known  as  "thunderheads." 

Stratus. — Stratus  cloud  is  a  widely  extended  sheet  of  uniform 
cloud,  named  from  its  resemblance  to  the  regular  arrangement  of  a 
stratum  of  rock  or  clay.  It  is  a  fog  lifted  up  floating  in  the  air.  It 
is  the  lowest  of  all  forms  of  clouds,  the  height  not  being  more  than 
1,000  to  2,000  feet.  It  is  essentially  a  cloud  of  the  night,  forming  by 
radiation  of  heat  from  the  lower  layers  of  the  air. 

Cirro-cumulus. — Cirro-cumulus  is  a  broken  layer  of  cloud  made  of 
little  bunches  of  whitish  cloud,  elliptical  or  elongated  patches  with 
somewhat  regular  interstices  between  them  without  any  shading  of 
light.  It  is  mostly  visible  near  sunset,  but  always  high  up  in  the 
sky,  at  a  height  of  12,000  to  22,000  feet.  It  is  denser  than  the  cirrus 
and  of  a  dark  "tone."  This  is  the  form  of  cloud  to  which  the  line  of 
poetry  applies: 

"  The  beauteous  semblance  of  a  flock  at  rest." 

Cirro-stratus. — Cirro-stratus  is  a  thin  veil  of  widely  extended  cir- 
rus. It  is  a  high  cloud.  At  times  it  is  largely  composed  of  ice 
particles,  as  shown  by  the  colored  rings  seen  around  the  sun  and 


56 

moon  viewed  through  it.  Lunar  halos  are  frequent  with  this  form 
of  cloud. 

Cirro-stratus  is  also  at  times  a  condensed  and  developed  form  of 
cirrus  in  which  the  streakiness  is  very  marked,  on  account  of  the 
great  extent  of  cirrus.  At  times  ribs  of  cirro-stratus  stretch  from 
a  point  on  the  horizon  to  a  point  directly  opposite.  This  is  an  effect 
of  perspective,  the  streaks  appearing  wider  apart  at  the  zenith. 
This  form  of  cloud  is  sometimes  known  as  "polar  bands."  The 
stripes  or  ribs  are  sometimes  made  up  of  cross-bar  patches  and  then 
are  said  to  be  "striated."  The  fibres  of  cirro-stratus  sometimes 
interlace  and  have  a  reticulated  appearance  like  woven  cloth. 

Cumulo-stratus. — Cumulo-stratus  is  composed  of  great  masses  of 
dense  dark  cloud,  usually  covering  the  whole  sky.  It  prevails  at  a 
height  of  5,000  to  10,000  feet.  It  is  essentially  a  cloud  of  the  night 
and  the  cold  season.  The  darker  forms  of  cumulo-stratus  sometimes 
present  an  unduiatory  aspect,  especially  toward  the  horizon,  an  effect 
of  perspective.  This  is  sometimes  called  "roll-cumulus."  In  high 
latitudes  in  winter  thick  masses  of  this  cloud  obscure  the  sky  at 
times  for  weeks. 

Nimbus. — Nimbus  is  a  dense  thick  layer  of  dark  cloud,  void  of  shape 
or  form,  without  definite  edge,  and  of  tattered,  ragged  outline,  from 
which  rain  or  snow  is  continuously  falling. 

Scud  is  the  small  detached  scurrying  masses  of  cloud  in  advance 
of  and  lower  down  than  the  main  nimbus  cloud. 

Cloud  observations  consist  in  estimating  the  part  of  the  sky  cov- 
ered at  any  time  by  clouds,  the  form  of  the  clouds,  and  their  direction 
and  rate  of  motion.  The  form  of  cloud  prevailing  indicates  some- 
thing as  to  the  condition  of  the  upper  air;  their  movement  indicates 
the  direction  and  rate  of  motion  of  the  upper  currents. 

The  proportion  of  sky  covered  by  clouds  is  determined  by  inspec- 
tion in  tenths  of  the  whole  visible  firmament.  A  sky  wholly  over- 
cast is  recorded  as  10;  a  perfectly  clear  sky  is  indicated  by  o.  Only 
an  approximation  to  the  true  amount  of  sky  covered  can  be  made 
by  estimation.  To  assist  in  this  estimation  it  maybe  useful  to  note 
that  the  sky  will  be  divided  into  10  equal  parts,  as  regards  angular 
magnitude  as  seen  by  an  observer,  by  lines  from  five  equi-distant 
points  on  the  horizon  to  the  zenith  and  by  a  horizontal  circle  at  one- 
third  of  the  distance  from  the  horizon  to  the  zenith. 

Sometimes  estimates  of  cloudiness  are  made  on  a  scale  of  o  to  4,  or 
quarters  of  the  sky  covered. 

The  form  of  cloud  is  noted  which  is  considered  to  be  the  nearest 
to  one  of  the  forgoing  types  described.  The  purely  typical  forms 
are  of  relatively  rare  occurrence. 

Observation  of  the  direction  of  motion  of  the  clouds  is  beset  with 
great  difficulty  in  most  cases,  from  the  fact  that  the  form  of  cloud 
is  continually  changing,  and  often  from  the  uniformity  of  texture  it 


57 

is  impossible  to  identify  a  point  on  a  cloud  for  comparison  with  some 
motionless  object  on  the  surface  of  the  earth.  In  the  case  of  clouds 
some  distance  from  the  zenith,  the  direction  of  motion  observed  must 
be  made  free  of  the  effects  of  perspective. 

CLOUD  MOVEMENT.  ' 

The  great  difficulty  in  getting  at  the  true  direction  of  cloud  move- 
ment lies  in  the  effect  of  perspective.  The  following  device  obviates 
'this: 

"  Set  up  a  pointed  pole,  reaching  6  or  8  feet  above  the  observer's 
head,  and  through  the  top,  an  inch  or  so  below  the  point,  fix  two 
stout  cross-wires,  or  thin  iron  rods,  set  truly  by  compass  to  the  four 
cardinal  points.  The  space  around  the  pole  must  be  sufficiently 
open  to  allow  of  a  good  view  of  the  sky  in  all  directions.  Let  the 
observer  then  station  himself  at  such  a  distance  from  the  pole,  and 
in  such  a  position  that  some  recognizable  portion  of  a  cloud  appears  to 
move  vertically  upward  from  the  top  of  the  pole  or  vertically  down- 
ward toward  it.  The  direction  of  the  pole  from  the  observer's 
position  (which  may  be  judged  of  accurately  by  means  of  the  cross- 
wires  on  the  top)  is  the  direction  of  the  cloud's  true  movement. 
With  a  little  care  in  selecting  the  position  the  pole  may  be  dispensed 
with,  as  any  pointed  object  will  serve  the  purpose,  provided  the 
observer  has  previously  acquainted  himself  accurately  with  the 
points  of  the  compass.  The  velocity  of  cloud  movement  may  be 
measured  in  favorable  situations  by  observing  the  time  that  the 
shadow  takes  to  traverse  a  certain  space  of  country,  the  extent  of 
which  is  accurately  known/' 

The  direction  from  which  the  clouds  are  moving  should  be  recorded 
to  8  points  N.,  NE,,  E.,  etc.  The  velocity  may  be  recorded  by  the 
letters  s  (slowly)  and  r  (rapidly)  placed  as  an  exponent  to  the  direc- 
tion thus,  Nr,  S8. 

Haze,  smoke,  and  fog  should  be  recorded  as  such,  with  the  addition 
of  the  words  "  light  "  or  "  dense,"  as  the  case  may  be. 

EVAPORATION. 

Depth  of  evaporation  in  inches  from  a  water  surface  is  measured 
by  means  of  an  evaporometer  or  atmometer.  A  common  form  con- 
sists of  a  round  dish  i  foot  in  diameter  and  4  inches  deep.  The 
depth  of  evaporation  is  ascertained  from  the  loss  of  weight  by  means 
of  weighing  at  different  times.  Sometimes  this  is  observed  by  means 
of  a  graduated  scale  inside  the  dish,  so  arranged  that  the  height  of 
water  surface  can  be  read  at  any  time.  Sometimes  the  scale  is 
inclined  so  as  to  give  a  magnified  reading  of  the  fall  of  surface. 

The  vessel  is  kept  in  the  open  air  and  free  from  the  direct  rays  of 
the  sun.  Readings  of  the  scale  are  made  daily.  The  differences  of 
readings  give  the  depth  of  evaporation  in  the  intervening  time. 


58 

The  dish  has  to  be  protected  from  wind  and  rain,  which  interfere 
with  exact  observation. 

The  Piche  evaporometer,  shown  in  Fig.  20,  consists  of  a  glass  tube 
about  9  inches  long  and  0.4  of  an  inch  internal  diameter,  closed  at 
the  top  and  open  below,  graduated  to  show  equal  volumes 
contained.  It  is  filled  with  water  and  the  end  covered 
with  a  paper  disk  1.2  inches  in  diameter,  held  in  place  by 
a  metal  plate  attached  to  a  spring  on  a  slitted  brass 
collar  moving  easily  along  the  tube.  The  water  flows 
down  to  the  paper,  from  which  it  evaporates  both  on 
the  upper  and  under  sides.  The  amount  of  evaporation 
in  a  given  time  is  obtained  by  taking  the  difference  in 
the  readings  of  the  top  of  the  column  of  water  in  the 
tube. 

A  paper  surface  gives  off  about  one-third  more  water 
in  a  given  time  than  an  equal  extent  of  water  surface 
in  a  dish. 

With  a  wind  velocity  of  5  miles  an  hour  the  evaporation 
is  2.2  times  as  great  as  in  a  calm  ;  with  10  miles,  3.8; 
with  15  miles,  4.9;  with  20  miles,  5.7;  with  25  miles,  6.1; 
with  30  miles,  6.3. 

RIVER  GAUGE. 

A  river  gauge  is  a  device  for  measuring  the  vertical 
height  of  a  river  surface  above  some  arbitrarily  selected 
plane,  usually  at  or  somewhere  near  the  level  of  the 
lowest  water  that  occurs  at  the  place. 

When   possible,  without  too  great  expense,   a   river       FIG.  20. 
gauge  is  made  vertical.     It  consists  of  a  plank  or  planks  2   inches 
thick  and  8  to   12  inches  wide,  fastened  to  a  bridge  pier  or  piling, 
and  of  sufficient  length  to  cover  the  greatest  range  in  height  of 
water  ever  likely  to  occur. 

The  plank  is  graduated  to  feet  and  half  feet  usually,  and  the 
height  of  surface  of  water  on  it  can  be  read  by  estimation  to  the 
nearest  tenth  of  a  foot. 

With  the  zero  of  the  gauge  graduation  at  or  near  low  water,  the 
reading  of  the  water  surface  on  the  gauge  at  any  time  is  called  the 
stage  of  the  river. 

When  a  gauge  has  to  be  read  on  a  bridge  pier  by  an  observer  on 
shore  at  a  distance  of  50  or  TOO  feet,  in  order  to  render  the  various 
foot  and  half-foot  marks  the  more  conspicuous  and  legible  and 
readily  recognizable,  the  gauge  is  painted  in  alternate  patches  or 
triangles  of  black  and  white,  according  to  some  such  design  as  that 
shown  in  Fig.  21. 

To  prevent  obscuring  of  the  foot  and  half-foot  marks  by  weather- 
ing, they  are  indicated  on  the  plank  by  rows  of  copper  tacks  driven 
into  the  plank. 


59 


Only  the  whole  foot  marks  are  numbered,  and  this  is  likewise 
done  in  copper  tacks. 

Sometimes  a  river  gauge  consists  of  a  strip  of  the  surface  of  a 
stone  pier  dressed  down  to  a  smooth  surface  so  as  to  receive  the 
marking  and  numbering  of  gauge. 

When  a  river  gauge  cannot  be  set  vertically  on  a  bridge  pier,  it 
is  laid  along  the  bank  according  to  the  slope  of  the  ground.  It 
should  then  be  made  of  logs  or  heavy  timbers  6 
inches  by  i  foot  imbedded  in  the  ground,  with  the 
top  surface  even  with  the  surface  of  the  ground  in 
the  river  bank.  Strap  iron  X  °f  an  incn  thick  and 
2  inches  wide  is  spiked  along  the  top  surface,  and 
[on  it  are  cut  the  foot  marks  and  intermediate  half 
foot  or  tenths  of  foot  marks  and  the  numbering  of 
the  feet  showing  various  heights  above  the  zero  of 
the  gauge.  The  foot  marks  on  a  gauge  of  this  kind 
must  be  accurately  located  by  a  civil  engineer  by 
means  of  a  spirit  level. 

Sometimes  a  very  substantial  inclined  gauge  is 
made  of  lengths  of  block  stone  with  bars  of  railroad 
iron  inlaid,  on  which  the  foot  marks  are  cut. 

Sometimes  a  gauge  is  made  of  isolated  stone  piers 
buried  in  the'ground  along  the  bank,  the  top  surface 
showing  the  height  in  feet  above  an  assumed  low- 
water  plane. 

It  is  not  always  possible  at  the  time  of  setting  a 
gauge  to  put  the  zero  of  the  graduation  at  the  exact 
level  of  the  lowest  water  apt  to  occur. 

When  a  stage  of  water  below  the  zero  occurs,  it 
is  read  as  a  minus  stage.     It  is  desirable  that  the 
zero  shall  be  put  so  low  that  this  will  never  occur,  as 
the  minus  sign  is  apt  to  lead  to  confusion,  the  same 
FIG.  21.          numerical  stages  indicating  very  different  levels  of 
water,  whether  plus  or  minus. 

A  gauge  once  established  and  a  long  record  of  readings  made,  it 
is  not  advisable  to  ever  change  the  zero  of  the  gauge.  It  is  not 
customary  to  change  it,  even  if  there  does  occur  a  stage  of  water 
lower  than  any  before  known. 

For  the  purpose  of  ascertaining  from  time  to  time  any  changes 
that  may  occur  in  the  level  of  the  zero  of  a  gauge  or  any  of  its 
marks,  a  bench  mark  is  established  close  by  the  gauge  or  somewhere 
in  its  vicinity. 

A  bench  mark  consists  of  some  accessible,  presumably  permanent 
point  or  surface,  the  difference  in  level  between  which  and  the  zero 
or  some  other  mark  on  the  gauge  is  known  by  actual  leveling  be- 


60 

tween  the  two  by  means  of  a  spirit  level.  When  this  difference  is 
found  to  vary  it  is  presumed  it  indicates  a  corresponding-  change  in 
the  level  of  the  marks  of  the  gauge,  requiring  adjustment  of  the 
gauge  or  correction  of  its  readings. 

A  bench  mark  is  essential  in  case  a  river  gauge  is  to  be  repaired 
or  renewed,  in  order  to  set  the  new  gauge  at  exactly  the  same  level 
as  it  was  before. 

On  a  bridge  pier  the  top  surface  of  the  largest  stone  accessible 
in  the  top  course  of  masonry  is  often  used  as  a  bench  mark.  Some- 
times a  bench  mark  is  the  top  surface  of  a  large  stone  buried  in  the 
ground  especially  for  the  purpose  of  establishing  a  permanent 
surface.  Prominent  surfaces  in  stone  buildings  are  good  places  for 
permanent  bench  marks.  A  copper  bolt  set  in  the  stone  wall  of 
some  public  building,  such  as  a  custom  house,  post  office,  or  city 
hall,  is  a  common  -device  for  a  bench  mark  in  a  large  city. 

STATE  OF  THE  WEATHER. 

The  weather  is  recorded  clear  when  the  sky  is  -^0-  or  less  obscured; 
fair,  when  the  sky  is  from  -fo  to  ^-  obscured;  cloudy,  when  the  sky 
is  more  than  -£$  obscured;  light  rain  (//.  r.),  when  there  is  light  rain; 
heavy  rain  (hy.  r.\  when  there  is  heavy  rain;  in  like  manner  with 
light  and  heavy  snow,  substituting  s  for  r;  fog,  haze,  smoke,  accord- 
ing as  these  are  predominant. 

FROST. 

Occurrence  of  first  and  last  frost  of  any  growing  season  should 
be  specially  noted,  as  well  as  all  killing  frosts  during  the  same. 

CORONA. 

These  must  be  distinguished  from  halos.  Coronae  are  very  common, 
especially  around  the  moon,  and  are  produced  by  the  rays  passing 
through  a  thin  layer  of  cloud.  Sometimes  as  many  as  three  small 
concentric  circles  may  be  seen  whose  diameters  are  in  the  ratio  of 
1:2:3.  They  are  frequently  colored,  red  being  the  outside  color. 
These  colors  are  not  the  pure  colors  of  the  spectrum,  but  rather  those 
of  the  opal,  and  are  caused  by  interference  and  not  refraction.  A  solar 
corona  is  not  often  visible  on  account  of  the  dazzling  brightness  of 
the  sun,  but  it  may  often  be  seen  by  viewing  the  sun  through  colored 
glass,  or  noticing  its  reflection  in  water. 

Halos  are  large  circles  of  45°  or  90°  in  diameter.  That  is,  the 
diameter  is  equal  to  one-eighth  or  one-fourth  the  circumference  of 
the  horizon.  Both  are  seldom  seen  at  the  same  time.  The  colors 
are  very  feeble,  generally  approaching  whiteness.  Halos  arise  from 
the  presence  in  the  atmosphere  of  minute  prisms  of  ice,  and  are  due 
to  refraction  of  light.  Sometimes  the  halo  is  intensified  into  two 
bright  spots,  one  on  each  side  of  the  central  luminary.  These  are 


61 

called  "parhelia"  or  " paraselenae "  (mock  S4ins  or  mock  moons), 
sometimes  sun-dogs.  Still  more  complicated  optical  phenomena  are 
sometimes  seen,  though  rarely,  except  in  high  latitudes. 

THUNDER  STORMS. 

Thunder  storms  six  hours  apart  may  be  considered  as  separate 
storms. 

Upon  the  occurrence  of  thunder,  give  as  nearly  as  possible  the 
times  of  first  and  loudest  thunder  and  duration  of  thunder  (being 
careful  to  note  a.  m.  or  p.  m.  if  the  hours  o  to  24.  are  not  used). 

Give  the  direction  from  which  the  storm  appears  to  be  coming,  as 
shown  by  threatening  sky,  lightning  flashes,  or  thunder  peals.  Also, 
the  direction  toward  which  it  goes. 

TORNADOES  AND  SAND  SPOUTS. 

All  the  meteorological  circumstances  attending  these  should  be 
minutely  noted,  viz:  the  course  of  the  barometer,  which  almost 
always  sinks  much  and  rapidly;  that  of  the  thermometer,  which 
usually  indicates  an  elevation  of  temperature;  the  region  of  the 
heavens  in  which  the  thunder  storm  frequently  accompanying  them 
is  formed;  the  form  and  color  of  the  clouds;  the  direction  and  in- 
tensity of  the  wind;  the  frequency,  intensity,  and  form  of  the  light- 
ning; finally,  the  apparent  shape  of  the  sand-spout,  its  variations,  its 
course,  and  its  effects  upon  the  trees  and  upon  the  ground,  the  oc- 
currence of  hail,  etc. 

INSTRUCTIONS  FOR  OBSERVING  AURORAS. 

Though  the  aurora  borealis  has  received  attention  during  a  con- 
siderable portion  of  the  last  two  centuries,  definite  information  is 
still  wanting  on  several  points  which  may  serve  as  the  basis  of  a 
sound  induction  as  to  its.  cause.  These  relate  particularly  to  the 
actual  frequency  of  its  appearance;  its  comparative  frequency  in 
the  different  months  of  the  year  and  the  different  hours  of  the  day; 
the  connection  of  its  appearance  with  other  atmospherical  phenom- 
ena; the  elevation  and  extent  of  visibility  of  the  arch;  and  whether 
the  same  or  different  phases  are  presented  to  individuals  at  different 
stations  at  the  same  moment  of  time;  finally  the  precise  influence  of 
the  arches,  streamers,  etc.,  on  the  magnetic  condition  of  the  earth; 
and  whether  any  unusual  electrical  effects  can  be  observed  during 
the  appearance  of  the  meteor. 

Auroral  phenomena  may  be  divided  into  the  following  classes: 

1.  A  faint  light  in  the  north,  without  definite  form  or  boundary. 

2.  A  diffused  light,  defined  by  an  arch  below. 

3.  Floating  patches  of  luminous  haze — sometimes  striated. 

4.  One  or  more  arches,  of  rainbow  form,  of  uniform  white  color, 


62 

retaining  the  same  apparent  position  for  a  considerable  time,  and 
varying  in  luminosity. 

5.  A  dark  segment  appearing  tinder  the  arch. 

6.  Beams,  rays,  streamers,  waves,  transverse  and  serpentine  bands, 
interrupted  or  checkered  arches,  frequently  tinged  with  color,  and 
showing  rapid  changes  in  form,  place,  and  color. 

7.  Auroral  corona,  or  a  union  of  beams  south  of  the  zenith. 

8.  Dark  clouds  accompanying  the  diffuse  light. 

9.  Sudden  appearance  of  haze  over  the  whole  face  of  the  sky. 
The  following  may  serve  as  a  scale  of  brightness:  (i)  Faint;  (2) 

Moderate;  (3)  Bright;  (4)  Very  bright. 

GENERAL    DIRECTIONS. 

1.  Make  a  regular  practice  of   looking  for  auroras  every  clear 
evening,  from  8  to  10  o'clock  or  later.     Record  the  result,  whether 
there  be  an  aurora  or  not. 

2.  Note  the  time  of  observation,  and  compare  the  watch  used  with 
a  correct  clock,  as  soon  after  as  is  convenient. 

3.  Make  a  record  of  the  latitude  and  longitude  of  the  station. 

4.  Note  the  class  to  which  the  auroral  phenomenon  belongs. 

5.  If  it  be  an  arch,  note  the  time  when  the  convex  side  reaches 
any  remarkable  stars,  when  it  passes  the  zenith,  disappears,  etc. 

6.  If  the  arch  be  stationary  for  a  time,  note  its  position  among 
the  stars,  so  that  its  altitude  may  be  determined. 

7.  If  it  be  a  streamer  or  beam,  note  its  position,  and  the  time  of 
its  beginning  and  ending. 

8.  If  motion  be  observed  in  the  beams,  note  the  direction,  whether 
vertically  or  horizontally,  to  the  east  or  west. 

9.  Note  the  time  of  the  formation  of  a  corona,  and  its  position 
among  the  stars. 

10.  Note  the  time  of  the  appearance  of  any  black  clouds  in  the 
north  near  the  aurora;  also,  if  the  sky  be  suddenly  overcast  with  a 
mist  at  any  time  during  the  auroral  display. 

11.  Give  the  direction  and  force  of  the  wind  at  the  time. 

12.  Note  if  any  electrical  effects  are  observed. 

13.  Note  the  effect  upon  a  delicately-suspended  magnetic  needle. 

14.  The  date,  hour,  and  minute  of  the  beginning  and  ending  of 
auroras  should  be  carefully  noted,  as  well  as  the  azimuth  and  alti- 
tude of  each  extremity  and  of  the  crown  of  any  arch  of  light,  and 
the  same  data  for  any  corona  or  glory  that  may  be  formed. 

When  the  observer  is  familiar  with  the  names  of  the  principal 
fixed  stars,  he  may  locate  the  arch  or  crown  by  reference  to  them, 
but  it  is  preferable  that  he  should  observe  directly  the  altitude  and 
azimuth. 

Altitudes  are  expressed  by  degrees  from  the  horizon  to  the  zenith. 


63 

If  any  circle  be  divided  into  three  hundred  and  sixty  parts,  and 
the  radial  lines  connect  these  parts  with  the  center,  each  pair  of 
lines  subtends  an  angle  of  i°;  the  fourth  'part  of  the  circle  will 
subtend  an  angle  of  90°  or  one  right  angle,  and  the  corresponding 
radii  are  perpendicular  to  each  other;  thus  the  zenith  (that  point 
of  the  heavens  immediately  above  the  observer)  is  90°  from  the 
horizon,  or,  in  other  words,  its  altitude  is  90°.  A  point  halfway  up 
from  the  horizon  to  the  zenith  has  an  altitude  of  45°. 

Azimuths  are  also  expressed  in  degrees,  but  are  measured  on  the 
horizontal  plane,  and  will  be  recorded,  as  is  done  in  astronomy,  from 
the  south  point  to  the  westward,  passing,  successively,  the  west, 
north,  and  east  points  of  the  compass  until  360°  have  been  passed 
over,  and  the  south  point  is  again  reached. 

Observers  should  be  particular  as  to  the  date  of  the  aurora;  and 
when  it  begins  in  the  evening  of  one  day  and  continues  into  the 
early  morning  of  the  next  day,  it  will  be  entered  as  occurring  on 
the  first  day,  but  its  details  will  be  given  in  the  record  as  occurring 
between  the  hours  of  its  actual  beginning  and  ending.  Thus,  an 
aurora  that  began  on  the  evening  of  the  i2th  of  January  and  con- 
tinued until  the  early  morning  of  the  i3th  would  be  entered  as  the 
aurora  of  the  i2th,  but  its  details  would  be  recorded  as  occurring, 
for  instance,  between  the  hours  of  10  p.  m.  of  January  12  and  2  a.  m. 
of  January  13. 

Professor  Olmstead,  in  a  paper  published  by  the  Smithsonian 
Institution,  classifies  different  auroras  as  follows: 

"  Class  I. — This  is  characterized  by  the  presence  of  at  least  three 
out  of  four  of  the  most  magnificent  varieties  of  form,  namely, 
arches,  streamers,  corona,  and  waves.  The  distinct  formation  of  the 
corona  is  the  most  important  characteristic  of  this  class;  yet,  were 
the  corona  distinctly  formed,  without  auroral  arches  or  waves,  or 
crimson  vapor,  it  could  not  be  considered  as  an  aurora  of  the  first 
class. 

"  Class  II. — The  combination  of  two  or  more  of  the  leading  charac- 
teristics of  the  first  class,  but  wanting  in  others,  would  serve  to 
mark  class  the  second.  Thus  the  exhibition  of  arches  and  streamers, 
both  of  superior  brilliancy,  with  a  corona,  while  the  waves  and 
crimson  columns  were  wanting,  or  of  streamers  with  a  corona,  or  of 
arches  without  a  corona,  without  streamers  or  columns  (if  such  a 
case  ever  occurs),  we  should  designate  as  an  aurora  of  the  second 
class. 

"  Class  III. — The  presence  of  only  one  of  the  more  rare  character- 
istics, either  streamers  or  an  arch,  or  irregular  coruscations,  but 
without  the  formation  of  a  corona,  and  with  but  a  moderate  degree 
of  intensity,  would  denote  an  aurora  of  the  third  class. 

"  Class  IV. — In  this  class  we  place  the  most  ordinary  forms  of  the 
aurora,  as  a  mere  northern  twilight,  or  a  few  streamers,  with  none 


64 

of  the  characteristics  that  mark  the  grander  exhibitions  of  the 
phenomenon." 

The  same  author  remarks: 

"On  the  evening  of  the  2yth  of  August,  1827,  after  a  long  absence 
of  any  striking  exhibition  of  the  aurora  borealis,  there  commenced 
a  series  of  these  meteors,  which  increased  in  frequency  and  mag- 
nificence for  the  ten  following  years,  arrived  at  a  maximum  during 
the  years  1835,  1836,  and  1837,  and,  after  that  period,  regularly 
declined  in  number  and  intensity  until  November,  1848,  when  the 
series  appeared  to  come  to  a  close.  The  recurrence,  however,  of 
three  very  remarkable  exhibitions  in  September,  1851,  and  of  another 
of  the  first  class  as  late  as  February  19,  1852,  indicates  that  the  close 
was  not  so  abrupt  as  was  at  first  supposed;  but  still  there  was  a  very 
marked  decline  in  the  number  of  great  auroras  after  1848,  and  there 
has  been  scarcely  one  of  the  higher  class  since  1853. 

EARTHQUAKES. 

Immediately  upon  the  occurrence  of  a  shock  it  is  desired  that  the 
observer  will  collect  such  information  as  will  enable  him  to  answer 
the  questions  here  propounded,  and  transmit  his  answers  with  the 
least  practical  delay  to  the  Weather  Bureau  office. 

If  any  other  reliable  information  be  in  his  possession,  it  is  desired 
that  he  will  communicate  it  at  the  same  time.  It  is  extremely 
desirable  that  the  nearest  possible  approximation  should  be  made 
to  the  exact  time  of  occurrence,  and  this  should  be  the  first  care  of 
the  observer  who  may  feel  the  shock.  If  the  only  time-piece  is  an 
ordinarily  good  watch  or  house  clock,  it  would  be  advisable  to  com- 
pare it  with  standard  time  at  the  earliest  practicable  moment,  which 
can  now  be  done  at  most  railway  stations. 


List  of  questions  to  be  answered  by  observers  of  an  earthquake  shock. 

Station 

Date _ 

1.  Was  ail  earthquake  shock  felt  at  your  place  on  the _ day  of 

,  18 ? 

(A  negative  answer  is  as  important  as  an  affirmative  one. ) 

2.  At  what  hour,  minuie,  and  second  of  standard  time  was  it  felt  f 

3.  How  long  did  its  perceptible  motion  continue  ? 

4.  Was  it  accompanied  by  any  unusual  noise  !  If  so,  describe  it 


5.  Was  more  than  one  shock  felt  j  if  so-  how  many  ?. 


6.  Which  of  the  following  measures  of  intensity  would  best  describe  what  hap- 
pened in  your  vicinity : 

No.  1. — Very  light ;  noticed  by  a  few  persons  ;  not  generally  felt. 

No.  2. — Light ;  felt  by  the  majority  of  persons ;  rattling  windows  and  crockery. 

No.  3  — Moderate ;  sufficient  to  set  suspended  objects,  chandeliers,  etc. ,  swinging, 
or  to  overthrow  light  objects. 

No.  4. — Strong ;  sufficient  to  crack  the  plaster  in  houses,  or  to  throw  down  some 
bricks  from  chimneys. 

No.  5. — Severe  j  overthrowing  chimneys  and  injuring  the  walls  of  houses. 


7.  Do  you  know  of  any  other  cause  for  what  happened  than  earthquake  ? 


Signature 

65 


GENERAL  PHENOMENA  OF  CLIMATE. 

Information  of  a  general  character  relating  to  the  growth  of 
plants  will  be  of  value  in  compiling  the  climatology  of  a  district. 

It  is  suggested  that  where  voluntary  observers  can  do  so  the  fol- 
lowing be  included  in  their  records  : 

Time  of  plowing  in  the  spring. 

Time  of  planting  various  crops. 

Time  of  appearance  of  same  above  ground. 

Time  of  flowering  of  strawberries,  currants,  raspberries,  apples, 
plums,  and  other  fruit. 

Time  of  commencement  of  haying. 

Time  of  commencement  of  harvesting  the  various  cereals. 

Time  of  ripening  of  various  fruits. 

Time  of  first  killing  frost  in  fall. 

Time  of  last  killing  frost  in  spring. 

Time  of  sowing  fall  wheat. 

Time  of  appearance  of  earliest  shoots  of  same  above  ground. 

Time  of  last  snow  on  ground. 

The  depth  of  snow  on  ground  on  the  last  day  of  each  winter 
month. 

The  time  of  migration  of  wild  fowl  and  birds,  the  nights  north 
and  south. 

The  time  of  leafing  and  fall  of  leaves  in  deciduous  forests. 

The  date  of  breaking  up  of  ice  in  large  rivers  and  bays. 

The  date  of  greatest  rise  and  lowest  water  in  important  streams. 

Voluntary  observers  are  requested  to  include  in  their  monthly 
reports  all  reliable  information  relative  to  the  destruction  of  life 
and  property  coming  to  their  knowledge,  due  to  storms,  classifying 
it,  as  far  as  possible,  as  indicated  in  the  following  table  : 


I 

I 

| 

it 

P, 

ill 

S'fl 

-3 

i 

0^ 

1 

•3 

o 

tOT3~'o 

•°  g'-o'o 

*  § 

«i 

r 

i 

8 

1   I! 

slil 

if 

3 

3  £  o 

s  s  S  o 

Date  of  storms. 

Nature  of  ston 
northeast  gal 

Section  of  coun 
by  storm. 

Number  and  u 
sons  killed. 

Number  and  n 
sons  injured. 

Number  and  n 
sels  lost  or  da 
estimated  am 

Number  of  ho 
and  other  b 
stl'oyed  or  da 
estimated  am 

Estimated  amo 
age  to  propei 

Number  of  ani 
and  estiniatec 

• 

66 


SPECIAL  DIRECTIONS  TO  THE  VOLUNTARY  OBSERVERS  OF  THE 
WEATHER  BUREAU. 

In  the  reduction  of  the  meteorological  records  presented  to  this 
office,  much  additional  labor 'has  resulted  from  the  occasional  omis- 
sion in  the  records  of  some  impoitant  facts,  and  in  want  of  perfect 
uniformity  in  noting  the  phenomena.  To  insure  uniformity  in  the 
records,  attention  is  called  to  the  following  remarks. 

1 .  Failure  to  record  latitude  and  longitude,  name  and  station  of  the  observer •, 
and  date  on  each  sheet,  the  observer  probably  supposing  it  sufficient  to 
insert  them  once  on  the  first  sheet  sent,  and  so  omitting  them  after- 
wards.    This  often  renders  it  necessary  to  search  back  through  all 
the  series  of  registers  to  some  one  that  contained  them — perhaps  in 
a  former  year.     They  should  be  inserted  on  every  sheet. 

2.  Designating   the  same  place   by  different  names,  thus  rendering   it 
impossible  to  distinguish  whether  it   is  one   place  or  two,  unless 
by  noticing  the  similarity  in  the  name  of  the  observer  or  in  the 
latitude  and  longitude.     Such  changes  of  name  should  be  avoided 
when   practicable,  and   when   necessarily   made,  special   attention 
should  be  called  to  it. 

3.  Diversity  in  the  mode  of  recording  the  barometer,  as  follows  : 

(a)  Integers  record  in  full,  thus,  29.35.     (THIS  is  the  proper  mode.) 
(£)  Integers  omittted  when  the  same  as  in  the  entry  next  above, 
thus,  38. 

(c)  Integers  omitted  when  the  same  as  in  the  entry  next  to  the  left. 

(d)  Integers  omitted  when  the  same  as  in  the  entry  next  preced- 

ing in  the  order  of  time. 

(e)  Integers  omitted,  except  where  they  are  different  from  the 

usual  ones  at  the  place  of  observation. 
(/)  Integers  inserted  occasionally  and  apparently  without  any 

system  whatever. 
(g)  A  constant  suppressed,  and  the  excess  or  deficiency  recorded, 

as  +  or  — . 
The  proper  mode  is  that  indicated  by  (a). 

4.  Diversity  in  the  mode  of  recording  the  thermometer,  when  it  is  below 
zero,  as  follows : 

(a)  Indicated  by  the  sign  —  placed  before  it,  thus,  — 16°. 

(This  is  the  proper  mode). 

(b)  Indicated  by  the  same  sign  placed  after  it,  thus,  16°— 

(c)  Indicated  by  writing  it  under  a  zero,  thus,  -— . 

(d)  Indicated  by  writing  it  after  a  zero,  with  a  comma  between, 

thus,  0,1 6°, 

(e)  Indicated  by  the  word  "below,"  or  the  abbreviation  b  written 

before  or  after  it,  thus,  16°  below,  i6°b,  b  16°,  or  below  16°. 
The  first  (a)  is  the  proper  mode. 
9377 5  67 


68 

• 

5.  Departure  from  the  printed  instructions  in  recording  the  degree  of  cloudi- 
ness, some  observers  reversing  the  figures  and  using  10  to  denote  a 
clear  sky,  and  o,  one  entirely  overcast;    and  others  omitting  the 
record  altogether  in  the  columns  of  cloudiness  when  the  sky  is  clear, 
and  in  place  of  it  sometimes  inserting  the  word  "  clear  "  in  the  columns 
of    "remarks,"    or  elsewhere.     Both  lead  to  error,  and -should  be 
avoided;  the  zero  should  always  be  inserted  "in  the  narrow  column," 
as  directed,  when  the  sky  is  clear. 

6.  Diversity  in  the  use  of  the  character  (0)  in  recording  the  motion  of  the 
clouds,  as  follows: 

(a)  Used  to  signify  a  calm,  or  that  there  is  no  perceptible  motion. 

(This  is  the  correct  use]. 

(b)  Used  to  signify  that  the  sky  is  clear,  instead  of  inserting  it  in 

the  proper  column. 

(c)  Used  to  signify  that  no  observation  was  taken. 

(d)  Used  to  signify  that  the  direction  in  which  the  upper  current 

was  moving  could  not  be  determined  on  account  of  the  sky 
being  either  perfectly  clear  or  entirely  overcast. 
The  first  (a)  is  the  correct  use. 

7.  Want  of  full  and  proper  records  of  the  direction  of  the  wind,  some 
observers  recording  the  direction  only  after  each  change,  and  then 
omitting  it  so  long  as  it  continues  the  same,  merely  inserting  a  figure 
to  denote  the  force.     It  is  better  to  make  the  record  in  full.     Other 
observers  record  the  direction  toward  which  the  wind  or  clouds  are 
moving  instead  of  indicating  that  from  which  they  come.     A  WIND 
from  the  north,  or  CLOUDS  moving  from  the  north,  are  to  be  denoted 
by  N,  and  from  the  south  by  S,  etc. 

8.  Different  kinds  of  thermometers  or  different  exposures  used  for  the  dry 
and  wet  bulb  thermometers,  so  that  the  observations  are  not  comparable 

•readily,  if  at  all. 

9.  Diversity  in  the  use  of  the  dash  and  the  sign  ("),  as  follows: 

(a)  To  signify  that  the  entry  next  above  is  to  be  repeated. 

(b)  To  signify  that  the  entry  next  to  the  left  is  to  be  repeated. 

(c)  To  signify  that  the  entry  next  preceding  in  the  order  of  time 
is  to  be  repeated. 

(d)  To  signify  nothing  at  all,  but  merely  to  fill  a  blank. 

The  use  of  these  characters  has  caused  much  trouble  in  the  re- 
duction, and  the  true  remedy  would  be  to  avoid  them  altogether,  by 
making  each  record  complete  in  itself. 

10.  Illegibility  of  the  records,  either  from  defective  chirography  or 
from  being  entered  in  pencil  marks  and  partly  erased. 


INSTRUCTIONS  IN  THE  USE  OF  THE  TABLES. 

TABLE  I. DEW  POINT. 

In  Table  I  are  given  the  temperatures  of  the  dew  point  as  depend- 
ent on  the  reading  of  the  dry  and  wet  bulb  thermometers.  The 
values  apply  to  roof  shelters  and  whirled  wet  and  dry  bulb  ther- 
mometers, but  may  be  used  without  much  error  for  all  shelters  well 
ventilated.  The  formula  from  which  the  table  is  derived  (Ferrel's) 
for  dew  points  below  32°  is: 
p=p7 — 0.000360  P  (t — t') (14-0-00065  t'),  and  for  dew  points  above  32°: 

t  —  t/ 
p—p'— 0.000367  P  (t— t')(i  + ),  in  which, 

p= vapor  pressure  of  saturation  at  the  dew  point  temperature. 
p'= vapor  pressure  of  saturation  at  the  wet  bulb  temperature. 
P=barometric  pressure  taken  as  29.4  inches  in  computing  the  tables, 
t  =dry  thermometer. 
t'=wet  thermometer. 

Example. 

To  find  the  dew  point  for  the  reading  of  the  dry  bulb  6o°.o  and 
the  wet  bulb  48°.o,  entering  the  table  on  page  75  with  t=6o  and 
t— t'  =  i2,  we  find  the  dew-point  35°. 

There  is  a  small  correction  to  be  applied  to  the  value,  if  great 
accuracy  is  desired,  when  the  barometric  pressure  differs  from  29.4 
inches.  This  is  obtained  by  the  use  of  Table  III,  in  connection  with 
Table  IV.  Having  found  the  dew  point,  the  vapor  pressure  corre- 
sponding to  it  is  taken  from  Table  IV;  in  the  example  above  for  35° 
it  is  0.203.  By  means  of  Table  III,  with  the  air  pressure,  say  27.0 
inches,  and  for  the  difference  of  dry  and  wet  bulb  12°,  the 
correction  to  the  vapor  pressure  is  found  to  be  +0.010.  The  vapor 
pressure  corrected  is  then  0.213,  an<^  by  Table  IV  the  corresponding 
dew  point  is  36°. 

TABLE    II. RELATIVE    HUMIDITY. 

In  Table  II  are  given  the  relative  humidities  corresponding  to  the 
reading  of  dry  and  wet  bulb  thermometers.  The  relative  humidity 
of  the  air  at  any  time  is  the  percentage  of  moisture  contained  in  the 
air  as  compared  with  the  whole  amount  it  is  capable  of  holding  for 
the  particular  temperature  at  the  time.  Air  containing  no  moisture 
is  at  zero  relative  humidity;  when  saturated  the  relative  humidity 
is  100. 

Example. 

To  find  the  relative  humidity  for  the  reading  of  the  dry  bulb  6o°.o 
and  the  wet-bulb  48°.o,  entering  the  table  on  page  85  with  t=6o°  and 
t— t'=i2,  we  find  the  relative  humidity  39  per  cent. 


70 

If  it  is  desired  to  correct  the  value  for  the  pressure  of  27.0  inches, 
the  procedure  is  as  follows: 

In  correcting  the  dew-point  for  pressure  the  corrected  value  in 
example  given  above  was  36°.  On  page  75  in  the  column  for  t— t'  =  i2, 
the  corrected  value  of  36°  is  one  line  lower  than  the  original  uncor. 
rected  value  35°.  Taking  the  corresponding  value  of  the  relative 
humidity  in  Table  II  on  page  85,  one  line  lower  than  39  in  the  column 
for  t— t'=i2,  we  have  the  corrected  relative  humidity  40  instead  of  39. 

Tables  III  and  IV  are  auxiliary  to  Tables  I  and  II. 

TABLE    IV. 

Table  IV  gives  the  vapor  pressure  at  saturation  for  various  tem- 
peratures. The  values  for  temperatures  above  32°  are  the  Regnault 
Broch  values;  below  32°  to  —60°  they  are  Marvin's  values  observed 
at  the  Weather  Bureau  office,  Washington  City  (see  report  of  Chief 
(Signal  Officer  for  the  year  1891). 

TABLE  V. — ABSOLUTE    HUMIDITY. 

Table  V  gives  the  weight  of  vapor  contained  in  the  air  per  cubic 
foot  of  air  when  saturated  at  different  temperatures. 

At  a  temperature  of  43°,  for  instance,  when  saturated,  the  weight 
of  vapor  contained  is  3.177  grains  of  water.  The  weight  of  vapor 
contained  in  the  air  at  different  vapor  pressures  is  nearly  propor- 
tional to  the  pressures. 

TABLE  VI. — REDUCTION  OF  BAROMETER  TO  32°. 

To  reduce  the  barometer  reading  to  the  temperature  of  32°,  enter 
Table  VI  with  the  observed  reading  and  the  temperature  as  shown 
by  the  attached  thermometer.  For  example,  suppose  the  reading  is 
28.50  and  the  attached  thermometer  73°,  the  reduction  is  —0.114,  or 
that  amount  is  to  be  subtracted  from  the  reading  to  give  the  pressure 
for  the  temperature  of  the  instrument  at  32°, 

The  reduction  is  additive  below  29°  and  subtractive  above. 

TABLE  VII. REDUCTION  OF  BAROMETER  TO  SEA  LEVEL. 

Table  VII  gives  the  approximate  reduction  to  be  applied  to  an 
observation  of  pressure  to  reduce  it  to  sea  level.  The  following 
example  will  show  its  use:  Suppose  the  height  of  the  station  is  670 
feet,  and  the  temperature  of  the  outside  air  obtained  from  the  dry  bulb 
thermometer  at  the  time  of  the  observation  is  45°,  then  the  reduction 
would  be  .737  or  .74;  this  amount  is  to  be  added  to  the  observed  ba- 
rometer reading  to  reduce  it  to  sea  level.  In  using  the  table,  observ- 
ers should  construct  one  in  manuscript,  after  learning  the  height  of 
the  station;  this  should  be  carried  out  to  every  degree  of  air  tem- 
perature for  the  height  of  the  station  as  found  in  the  left-hand 
column. 


71 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


^ 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

JS 

t 

0°.2 

0°.4 

0°.6 

0°.8 

to.0 

10.2 

10.4 

10.6 

10.8 

2°.0 

20.2 

2°.4 

20.6 

b 

—40 

—52 

9 

(t-f) 

—40 

39 

—50 

~£ 

—39 

—38 
—37 

—49 

—48 

£ 

001 

0°.2 

—38 

—37 

—36 

—46 

•"•* 

—36 

—35 

—  44 

—50 

—60 

—35 

34 

—43 

—58 

—49 

—58 

-34 

—33 

—42 

—55 

—48 

—56 

—33 

—32 

—40 

—52 

—47 

—54 

—32 

-31 

—38 

—49 

—46 

—53 

—31 

30 

—36 

—47 

—45 

—51 

—30 

—29 

—35 

—44 

—44 

—50 

—60 

—28 

33 

—42 

—56 

—43 

—49 

—58 

—28 

—27 

—32 

-40  -52 

—42 

—48 

—56 

—27 

—26 

—30 

-37 

—48 

-41 

-46 

—54 

—26 

—25 

—29 

—35 

-45 

—40 

—45 

—52 

—25 

—24 
23 

-28  |  -34 
—27   —32 

—43 
—40 

—58 
—53 

—39 
—38 

—44 
—43 

—50 
-49 

—24 
—23 

22 

—26  !  —30 

—37 

—49 

-37 

—42 

—48 

—22 

—21 

—25  —29 

—35 

—45 

—36 

—41 

—46 

—21 

—20 

—23  —28 

—33 

—41 

—55 

—35 

—39 

44 

—20 

—19 

18 

-22 
21 

—26 
—25 

—31 

—29 

—38 
—35 

—50 
—45 

—34 

—33 

—38 
—37 

—43 
—42 

—19 
—18 

17 

<20 

—23 

—27 

—3-2 

—41 

—55 

—32 

—35 

—40 

—17 

—16 

—19 

—22 

—26 

—30 

—37 

—49 

—31 

—34 

—38 

—16 

—15 
14 

—17 
16 

—20 
19 

—24 

22 

-28 
-26 

-34 
—31 

—44 
—39 

—52 

—30 

—33 

—36 

—15 
—14 

—13 
—12 

—15 
—  14 

—18 
—17 

—21 
—20 

—25 
—23 

29 

—27 

35 

—  :tt 

46 
—41 

—55 

-13 

—12 

—11 

—13 

—16 

—18 

—21 

—25 

—30 

—36 

—48 

—11 

—10 
g 

—12 
11 

—14 
13 

—17 
—15 

—20 
—18 

—23 
—21 

—27 
—25 

—33 
—30 

—42 
—37 

—58 
—48 

—10 
—  9 

—  8 

-  7 
—  6 

—10 

—  9 

—  8 

—12 
—11 

—10 

—14 
-13 

—12 

—17 
—15 
—14 

—20 

—18 
—16 

—23 

*>"! 

—19 

—27 
—25 
-23 

—33 

—30 

—27 

—42 
—36 
—32 

—58 
—48 
—41 

—56 

—  8 

-  7 
—  6 

K 

7 

—  8 

—10 

—12 

—15 

—17 

—21 

—24 

—29 

—35 

—47 

—  5 

7 

—  9 

—11 

—13 

—16 

—19 

—22 

—26 

—31 

—39 

—54 

—  4 

~~  0 

4 

g 

—  8 

—10 

—12 

—14 

—17 

—20 

—23 

—28 

—33 

44 

—  8 

0 

3 

5 

—  6 

—  8 

—10 

—12 

—15 

—18 

—21 

—24 

—29 

—36 

—48 

—  2 

—  L 

—  i 

—  2 

—  4 

—  5 

—  7 

—  9 

—11 

—13 

—16 

—18 

—22 

—26 

—31 

—39 

—  1 

1 

3 

4 

6 

7 

—  9 

—11 

—14 

—16 

—19 

-23 

—27 

—33 

0 

+  i 

1 

—  0 

1   1 

—  2 
1 

—  3 

2 

—  4 
3 

—  6 
—  5 

—  8 
—  6 

—10 
—  8 

—12 
—10 

—14 

—12 

—17 
—15 

—20 
—17 

—24 
—21 

—28 
—25 

+  1 

- 

1 

—  2 

3 

.—  5 

_  7 

—  8 

—10 

—13 

—15 

-18 

—21 

3 

4 

3 

+  l 

+  2 

0 

—  1 

—  2 

4 

—  5 

—  7 

—  9 

—11 

—13 

—16 

—19 

4 

5 

4 

3 

+  1 

0 

—  1 

2 

—  4 

—  5 

—  7 

—  9 

—11 

—14 

12 

—16 
14 

5 
6 

6 
7 

5 
g 

4 

5 

3 
4 

+  1 
3 

0 
+  l 

0 

—  1 

—  3 

—  4 

—  6 

—  8 

—10 

—12 

7 

g 

7 

6 

5 

4 

3 

+  1 

0 

—  1 

—  3 

—  4 

—  6 

—  8 

—10 

8 

9 

8 

7 

6 

5 

4 

3 

+  1 

0 

-  1 

—  3 

—  4 

—  6 

—  8 

9 

10 

9 

8 

7 

6 

5 

4 

3 

+  1 

0 

—  1 

—  3 

—  4 

—  6 

10 

11 

10 

9 

8 

7 

6 

5 

4 

3 

+  2 

0 

—  1 

—  3 

—  4 

11 

12 

11 

10 

9 

9 

8 

7 

5 

4 

3 

+  2 

0 

-  1 

—  2 

12 

13 

12 

11 

11 

10 

9 

8 

7 

6 

5 

3 

+  2 

+  1 

—  1 

13 

14 

13 

12 

12 

11 

10 

9 

8 

7 

6 

5 

4 

2 

+  1 

14 

15 

14 

13 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 

15 

16 

15 

15 

14 

13 

12 

11 

10 

10 

9 

8 

7 

5 

4 

16 

17 

16 

16 

15 

14 

13 

12 

12 

11 

10 

9 

8 

7 

6 

17 

18 

17 

17 

16 

15 

14 

14 

13 

12 

11 

10 

9 

8 

7 

18 

19 

18 

18 

17 

16 

15 

15 

14 

13 

12 

11 

10 

10 

9 

19 

20 

19 

19 

18 

17 

17 

16 

15 

14 

13 

13 

12 

11 

10 

20 

t. 

0°.2 

0°.4 

0°.6 

0°.8 

1°0 

10.2 

10.4 

10.6 

10.8 

2°.0 

2°.  2 

2°.4 

2°.  6 

t. 

Point. 


72 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


k 

0" 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

* 

I 

2°.  6 

2°.  8 

3°.0 

3°.  2 

3°.  4 

3°.  6 

3°.  8 

4°.0 

4°.  2 

4°.  4 

fc 

g, 

—  2 

—48 

2 

—  1 

—39 

—54 

—  1 

0 

—33 

—43 

0 

+  1 

—28 

—35 

-46 

+  1 

I  2 

—25 

—30 

—37 

—50 

2 

3 

—21 

—26 

—31 

—39 

—54 

3 

4 

—19 

—22 

—27 

—32 

—42 

—60 

4 

5 

—16 

—19 

—23 

—28 

—34 

—45 

5 

G 

—14 

—17 

—20 

—24 

—29 

—35 

—47 

6 

7 

—12 

—14 

—17 

—20 

—24 

—29 

—37 

—50 

7 

8 

—10 

—12 

—15 

—17 

—21 

—25 

—30 

—38 

—53 

8 

9 

—  8 

—10 

-12 

—15 

—18 

—21 

—25 

—31 

—39 

—55 

9 

10 

—  6 

—  8 

—10 

—12 

—15 

—18 

—21 

—26 

^31 

—40 

10 

11 

_  4 

—  6 

—  8 

—10 

—12 

—15 

—18 

—21 

—26 

—31 

11 

12 

—  2 

—  4 

—  6 

—  8 

-10 

—12 

—15 

—18 

—21 

—26 

12 

13 

_  1 

—  2 

—  4 

—  5 

—  7 

—  9 

—12 

—14 

—17 

—21 

13 

14 

+  1 

0 

—  2 

—  3 

—  5 

—  7 

—  9 

—11 

—14 

—17 

14 

15 

3 

+  1 

0 

—  2 

—  3 

—  5 

—  7 

—  9 

—11 

—14 

15 

16 

4 

3 

+  2 

0 

—  1 

—  3 

—  4 

—  6 

—  8 

—10 

16 

17 

6 

5 

3 

+  2 

+  1 

—  1 

—  2 

_  4 

—  6 

—  8 

17 

18 

7 

6 

5 

4 

2 

+  1 

0 

—  2 

—  3 

—  5 

18 

19 

9 

8 

7 

5 

4 

3 

+  1 

0 

—  l 

—  3 

19 

20 

10 

9 

8 

7 

6 

5 

3 

+  2 

+  1 

—  1 

20 

t. 

2°.G 

2°.  8 

3°.0 

3°.  2 

3°.  4 

3°.  6 

3°.  8 

4°.0 

4°.  2 

4°.  4 

t. 

t. 

4°.  6 

4°.  8 

5°.0 

5°.  2 

5°.  4 

5°.  6 

5°.  8 

6°.  0 

G°.2 

G°.4 

t. 

8 

8 

9 

9 

10 

—57 

10 

11 

—41 

—60 

11 

12 

—31 

—41 

—59 

12 

13 

-25 

—31 

—40 

—58 

13 

14 

—20 

—25 

—30 

—39 

—56 

14 

15 

—16 

—20 

—24 

—30 

—38 

—53 

15 

16 

—13 

—16 

—19 

—23 

—28 

—36 

—50 

16 

17 

—10 

—12 

-15 

—18 

—22 

—27 

—34 

—47 

17 

18 

—  7 

—  9 

[2 

—14 

—17 

—21 

—26 

—32 

—44 

18 

19 

-  5 

—  7 

—  9 

—11 

—14 

—17 

—20 

—25 

—30 

—40 

19 

20 

—  2 

—  4 

—  6 

—  8 

—10 

—13 

—16 

—19 

—is 

—29 

20 

t. 

4°.  6 

4°.  8 

5°.0 

5°.  2 

5°.  4 

5°.  6 

5°.  8 

6°.0 

6°.  2 

6°.  4 

t. 

73 

TABLE  I — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


1 

1 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

t 
(Dry  ther.) 

0°.5 

1°.0 

1°.5 

2°.0 

2°.  5 

3°.0 

3°.  5 

4°.0 

4°.  5 

5°.0 

5°.  5 

6°.0 

20 

18 

17 

15 

13 

10 

8 

5 

2 

—2 

—6 

-12 

—19 

20 

21 

19 

18 

16 

14 

12 

9 

7 

4 

0 

—4 

—  8 

—  15 

21 

22 

20 

19 

17 

15 

13 

11 

8 

6 

+2 

1 

—  6 

—11 

22 

23 

22 

20 

18 

16 

14 

12 

10 

7 

4 

+1 

—  3 

—  8 

23 

24 

23 

21 

19 

18 

16 

14 

11 

9 

6 

3 

—  1 

—  5 

24 

25 

24 

22 

21 

19 

17 

15 

13 

11 

8 

5 

+  2 

—  2 

26 

26 

25 

23 

22 

20 

18 

16 

14 

12 

10 

7 

4 

0 

26 

27 

26 

24 

23 

21 

20 

18 

16 

14 

11 

9 

6 

+  3 

27 

28 

27 

25 

24 

22 

21 

19 

17 

15 

13 

11 

8 

5 

28 

29 

28 

26 

25 

24 

22 

20 

19 

17 

14 

12 

10 

7 

29 

30 

29 

27 

26 

25 

23 

22 

20 

18 

16 

14 

11 

9 

30 

31 

30 

29 

27 

26 

24 

23 

21 

19 

18 

15 

13 

11 

31 

32 

31 

30 

28 

27 

26 

24 

22 

21 

19 

17 

15 

13 

32 

33 

31 

31 

29 

28 

26 

25 

23 

22 

19 

18 

16 

14 

33 

34 

32 

32 

30 

29 

27 

26 

24 

24 

21 

20 

18 

16 

34 

35 

33 

32 

31 

30 

29 

28 

26 

25 

23 

22 

20 

18 

35 

36 

35 

34 

32 

31 

30 

29 

27 

26 

24 

23 

21 

19 

36 

37 

36 

35 

33 

32 

31 

30 

28 

27 

26 

24 

22 

21 

37 

38 

37 

36 

34 

33 

32 

31 

30 

28 

27 

26 

24 

22 

38 

39 

38 

37 

35 

34 

33 

32 

30 

29 

28 

27 

25 

24 

39 

40 

39 

38 

36 

35 

34 

33 

31 

30 

29 

28 

26 

25 

40 

41 

40 

39 

37 

3(5 

35 

34 

32 

32 

30 

29 

28 

26 

41 

42 

41 

40 

39 

38 

36 

35 

34 

33 

31 

30 

29 

27 

42 

43 

42 

41 

40 

39 

37 

36 

35 

34 

32 

31 

30 

29 

43 

44 

43 

42 

41 

40 

38 

37 

36 

35 

33 

32 

31 

30 

44 

45 

44 

43 

42 

41 

40 

39 

37 

36 

34 

33 

32 

31 

45 

46 
47 

45 
46 

44 
45 

43 
44 

42 
43 

41 

42 

40 
41 

38 
40 

37 
39 

36 
37 

35 
36 

33 
34 

32 
33 

t? 

48 

47 

46 

45 

44 

43 

42 

41 

40 

38 

37 

36 

35 

48 

49 

48 

47 

46 

45 

44 

43 

42 

41 

39 

38 

a7 

36 

49 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

38 

37 

50 

51 

.  50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

39 

38 

51 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

40 

52 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

43 

42 

41 

53 

54 

53 

52 

51 

50 

50 

49 

47 

46 

45 

44 

43 

42 

54 

55 

54 

53 

53 

52 

51 

50 

49 

48 

47 

46 

44 

43 

55 

56 

55 

54 

54 

53 

52 

51 

50 

49 

48 

47 

45 

44 

56 

57 

56 

55 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

57 

58 

57 

56 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

58 

59 

58 

57 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

59 

60 

59 

58 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

60 

61 

60 

59 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

61 

62 

61 

60 

60 

59 

58 

57 

56 

55 

54 

53 

52 

52 

62 

63 

62 

61 

61 

60 

59 

58 

57 

56 

55 

55 

54 

53 

63 

64 

63 

62 

62 

61 

60 

59 

58 

57 

56 

56 

55 

54 

64 

65 

04 

63 

63 

62 

61 

60 

59 

59 

58 

57 

56 

55 

65 

66 

65 

64 

64 

63 

62 

61 

60 

60 

59 

58 

57 

56 

66 

67 

67 

615 

65 

64 

63 

62 

61 

61 

60 

59 

58 

57 

67 

68 

68 

67 

66 

65 

64 

63 

62 

62 

61 

60 

59 

58 

68 

69 

69 

68 

67 

66 

65 

64 

63 

63 

62 

61 

60 

59 

69 

70 

70 

69 

68 

67 

67 

66 

65 

64 

63 

62 

61 

61 

70 

71 

71 

70 

69 

68 

68 

67 

66 

65 

64 

63 

62 

62 

71 

72 

72 

71 

70 

69 

69 

68 

67 

66 

65 

64 

63 

63 

72 

73 

73 

72 

71 

70 

70 

69 

68 

67 

66 

66 

65 

64 

73 

74 

74 

73 

72 

71 

71 

70 

69 

68 

67 

67 

66 

65 

74 

75 

75 

74 

73 

72 

72 

71 

70 

69 

68 

68 

67 

66 

75 

70 

76 

75 

74 

73 

73 

72 

71 

70 

69 

69 

68 

67 

76 

77 

77 

76 

75 

74 

74 

73 

72 

71- 

70 

70 

69 

68 

77 

78 

78 

77 

76 

75 

75 

74 

73 

72 

71 

71 

70 

69 

78 

79 

79 

78 

77 

76 

76 

75 

74 

73 

72 

72 

71 

70 

79 

80 

80 

79 

78 

77 

77 

76 

75 

74 

73 

73 

72 

72 

80 

t. 

0°.5 

1°.0 

1°.5 

2°.0 

2°.  5 

3°.0 

3°.  5 

4°.0 

40.5 

5°.0 

5°.5 

6°.0 

<. 

74 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


t 
(Dry  ther.) 

Difference  between  the  dry  and  wet  thermometers  (<  —  t'). 

t 
(Dry  ther.) 

6°.0 

63.5 

7°.0 

7°.  5 

8°.0 

8°.  5 

9°.0 

9°.  5 

10°.  0 

10°.  5 

11°.  0 

11°.  5 

12°.  0 

19 

—25 

19 

20 

—19 

—32 

20 

21 

—15 

—24 

-47 

21 

22 

-11 

—19 

-31 

22 

23 

—  8 

—14 

-24 

—45 

23 

24 

—  5 

-10 

-18 

—30 

24 

25 

-  2 

—  7 

-13 

—22 

—42 

25 

26 

0 

—  4 

—  9 

-17 

—28 

26 

27 

+  3 

—  1 

-  6 

-12 

-20 

—37 

27 

28 

5 

+  1 

O 

—  8 

—15 

—25 

—54 

28 

29 

7 

4 

0 

—  4 

-10 

-18 

-32 

29 

30 

9 

6 

+  2 

-  2 

—  6 

-13 

-22 

—43 

30 

31 

11 

8 

5 

+  1 

—  3 

—  8 

-15 

-27 

31 

32 

13 

10 

7 

4 

0 

—  4 

—10 

-18 

—33 

32 

33 

14 

12 

9 

6 

+  3 

—  1 

—  6 

-12 

-22 

—44 

33 

34 

16 

14 

11 

8 

6 

+  2 

—  2 

Q 

-15 

-27 

34 

35 

18 

15 

13 

10 

8 

5 

+  1 

-  4 

-  9 

—18 

—32 

35 

36 

19 

17 

15 

12 

10 

8 

4 

0 

—  5 

—12 

-20 

—42 

36 

37 

21 

19 

17 

14 

12 

9 

6 

+  3 

-  2 

—  6 

—14 

-25 

-52 

37 

38 

22 

20 

19 

16 

14 

11 

9 

6 

+  2 

—  2 

—  8 

-16 

-29 

38 

39 

24 

22 

20 

18 

16 

14 

11 

8 

5 

+  1 

A 

-10 

-18 

3U 

40 

25 

23 

22 

20 

18 

16 

13 

11 

8 

4 

0 

-  5 

-12 

40 

41 

20 

25 

23 

21 

20 

17 

15 

13 

10 

7 

+  4 

—  1 

—  6 

41 

42 

27 

26 

24 

23 

21 

19 

18 

15 

12 

10 

7 

+  3 

-  2 

42 

43 

29 

27 

26 

24 

23 

21 

19 

17 

14 

12 

9 

6 

+  2 

43 

44 

30 

28 

27 

20 

24 

22 

20 

18 

16 

14 

12 

9 

6 

44 

45 

31 

30 

28 

27 

25 

24 

22 

20 

IS 

16 

13 

11 

8 

45 

46 

32 

31 

30 

28 

27 

25 

24 

22 

20 

18 

16 

13 

11 

46 

47 

33 

32 

31 

29 

28 

26 

25 

23 

22 

20 

18 

15 

13 

47 

48 

35 

33 

32 

30 

29 

28 

26 

25 

23 

21 

20 

17 

15 

48 

49 

36 

34 

33 

32 

31 

29 

28 

26 

25 

23 

21 

19 

17 

49 

50 

37 

35 

34 

33 

32 

31 

29 

28 

26 

24 

23 

21 

19 

50 

51 

38 

37 

36 

34 

33 

32 

31 

29 

28 

26 

24 

22 

21 

51 

52 

40 

38 

37 

36 

34 

33 

32 

30 

29 

28 

20 

24 

23 

52 

53 

41 

39 

38 

37 

36 

34 

33 

32 

30 

29 

28 

20 

24 

53 

54 

42 

41 

40 

39 

37 

36 

34 

33 

32 

30 

29 

27 

26 

54 

55 

43 

42 

41 

40 

39 

37 

36 

34 

33 

32 

30 

29 

28 

55 

50 

44 

43 

42 

41 

40 

39 

37 

36 

34 

33 

32 

30 

29 

56 

57 

46 

45 

44 

42 

41 

40 

39 

37 

36 

35 

33 

32 

•  30 

57 

58 

47 

46 

45 

44 

42 

41 

40 

39 

37 

36 

35 

33 

32 

58 

59 

48 

47 

40 

45 

44 

43 

41 

40 

39 

38 

36 

35 

33 

59 

60 

49 

48 

47 

46 

45 

44 

43 

41 

40 

39 

38 

36 

35 

60 

61 

50 

49 

48 

47 

40 

45 

44 

43 

42 

41 

39 

38 

36 

01 

62 

52 

51 

50 

49 

48 

47 

45 

44 

43 

42 

41 

39 

38 

62 

63 

53 

52 

51 

50 

49 

48 

47 

45 

44 

43 

42 

41 

39 

63 

64 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

43 

42 

41 

64 

65 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

43 

42 

65 

66 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

46 

45 

44 

66 

67 

57 

56 

55 

55 

54 

53 

52 

51 

50 

48 

47 

46 

45 

67 

68 

58 

57 

57 

56 

55 

54 

53 

52 

51 

50 

49 

47 

46 

68 

69 

59 

58 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

69 

70 

Gl 

60 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

70 

71 

62 

61 

60 

59 

58 

57 

56 

55 

55 

54 

53 

52 

51 

71 

72 

63 

62 

61 

60 

59 

59 

58 

57 

56 

55 

54 

53 

52 

72 

73 

64 

63 

62 

62 

61 

60 

59 

58 

57 

56 

55 

54 

53 

73 

74 

65 

64 

63 

63 

62 

61 

60 

59 

58 

57 

56 

55 

54 

74 

75 

66 

65 

64 

64 

63 

62 

61 

60 

59 

58 

57 

56 

56 

75 

76 

67 

66 

65 

65 

64 

63 

62 

61 

61 

60 

59 

58 

57 

76 

77 

68 

67 

67 

66 

65 

64 

63 

62 

62 

61 

60 

59 

58 

77 

78 

69 

68 

68 

67 

66 

66 

65 

64 

63 

62 

61 

60 

59 

78 

79 

70 

69 

69 

68 

67 

67 

GO 

65 

64 

63 

62 

61 

61 

79 

80 

72 

71 

70 

09 

68 

68 

67 

66 

05 

64 

63 

62 

62 

80 

t. 

6°.0 

6°.  5 

7°.0 

7°.  5 

8°.0 

8°.  5 

9°.0 

9°.  5 

10°.  0 

10°.  5 

11°.  0 

11°.  5 

12°.  0 

/. 

75 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


B 

M 

>-, 
£ 

Difference  between  the  dry  and  wet  thermometers  (t  —  t?). 

h 

d 

"| 

12°.  0 

12°.  5 

13°.  0 

13°.  5 

14°.  0 

14°.  5 

15°.  0 

15°.  5 

16°.  0 

16°.  5 

17°.  0 

17°.  5 

18°.  0 

40 

—12 

-22 

—44 

40 

41 

-  6 

—13 

—25 

41 

42 

—  2 

n 

—15 

—28 

42 

43 

+  2 

—  3 

—  8 

—17 

—33 

43 

44 

6 

+  1 

—  4 

—10 

—19 

—40 

44 

45 

8 

5 

0 

—  4 

—11 

—22 

—48 

45 

46 

11 

8 

+  * 

0 

—  5 

—13 

-24 

46 

47 

13 

10 

7 

+  3 

1 

—  6 

—14 

—27 

47 

48 

15 

12 

10 

6 

+  2 

—  2 

—  8 

—16 

—30 

48 

49 

17 

14 

12 

9 

6 

+  2 

—  3 

—  9 

-18 

—35 

49 

50 

19 

16 

14 

12 

9 

5 

+  1 

—  4 

—10 

—20 

—42 

50 

51 

21 

18 

17 

14 

11 

8 

5 

0 

—  5 

—12 

—22 

—52 

51 

52 

23 

21 

19 

16 

U 

11 

8 

+  4 

0 

—  6 

—13 

—25 

52 

53 

24 

22 

20 

18 

16 

14 

11 

8 

+  4 

—  1 

—  6 

—14 

—28 

53 

54 

26 

24 

22 

20 

18 

16 

13 

10 

7 

+  3 

—  2 

—  8 

—16 

54 

55 

28 

26 

24 

22 

20 

18 

16 

13 

10 

7 

+  3 

—  2 

—  8 

55 

56 

29 

27 

26 

24 

22 

20 

18 

15 

13 

10 

6 

+  2 

.—  2 

56 

57 

30 

29 

28 

26 

24 

22 

20 

18 

15 

13 

10 

6 

+  2 

57 

58 

32 

30 

29 

27 

26 

24 

22 

20 

18 

15 

12 

9 

6 

68 

59 

33 

32 

31 

29 

27 

26 

24 

22 

20 

18 

15 

12 

9 

59 

60 

35 

33 

32 

30 

29 

27 

26 

24 

22 

20 

18 

15 

12 

60 

61 

36 

35 

33 

32 

31 

29 

28 

26 

24 

22 

20 

18 

15 

61 

62 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

20 

18 

62 

63 

39 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

20 

63 

64 

41 

39 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

22 

64 

65 

42 

41 

40 

38 

37 

35 

34 

32 

31 

29 

28 

26 

24 

65 

66 

44 

43 

41 

40 

38 

37 

35 

34 

32 

31 

30 

28 

'  26 

66 

67 

45 

44 

43 

41 

40 

39 

37 

36 

34 

32 

31 

30 

28 

67 

68 

46 

45 

44 

43 

42 

40 

39 

38 

36 

34 

33 

31 

30 

68 

69 

48 

47 

46 

45 

43 

42 

40 

39 

38 

36 

34 

33 

32 

69 

70 

49 

48 

47 

46 

45 

43 

42 

41 

39 

38 

36 

35 

33 

70 

71 

51 

49 

48 

47 

46 

45 

43 

42 

41 

39 

38 

36 

35 

71 

72 

52 

51 

50 

49 

47 

46 

45 

44 

43 

41 

40 

38 

37 

72 

73 

53 

52 

51 

50 

49 

48 

46 

45 

44 

43 

41 

40 

38 

73 

74 

54 

53 

52 

51 

50 

49 

48 

47 

45 

44 

43 

41 

40 

74 

75 

56 

55 

54 

53 

52 

50 

49 

48 

47 

45 

44 

43 

42 

75 

76 

57 

56 

55 

54 

53 

52 

50 

49 

48 

47 

46 

45 

43 

76 

77 

58 

57 

56 

55 

54 

53 

52 

51 

50 

4!) 

48 

46 

45 

77 

78 

59 

58 

57 

56 

55 

54 

53 

52 

51 

50 

49 

48 

47 

78 

79 

61 

60 

59 

58 

57 

56 

55 

54 

53 

52 

51 

49 

48 

79 

80 

62 

61 

60 

59 

58 

57 

56    55 

54 

53 

52 

51 

50 

80 

t. 

12°.  0 

12°.  5 

13°.  0 

13°.  5 

14°.  0 

14°.  5 

15°.  0 

15°.  5 

16°.  0 

16°.  5 

17°.  0 

17°.  5 

18°.  0 

I. 

76 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


t 
(Dryther.) 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

2 

>-> 

Q_ 

18°.0 

19°.0 

20°.0 

21°.0 

22°.  0 

23°.0 

24°.0 

25°.0 

26°.0 

27°.0 

28°.0 

29°.0 

30°.  0 

55 

-8 

55 

66 

—2 

—19 

56 

57 

+2 

—10 

—48 

57 

58 

6 

3 

—22 

58 

59 

9 

+  1 

—12 

59 

60 

12 

5 

—  5 

-25 

60 

61 

15 

9 

0 

—14 

61 

62 

18 

12 

4-  5 

—  6 

—28 

62 

68 

20 

15 

9 

0 

—14 

63 

64 

22 

18 

12 

+  4 

—  6 

—32 

64 

65 

24 

20 

15 

9 

0 

—16 

65 

66 

26 

22 

18 

12 

-1-  4 

—  7 

—34 

66 

67 

28 

24 

20 

15 

9 

—  1 

-16 

67 

68 

30 

26 

23 

18 

12 

+  4 

—  7 

—37 

68 

69 

32 

28 

25 

20 

15 

8 

0 

—17 

69 

70 

33 

30 

27 

23 

19 

12 

+  5 

—  7 

—39 

70 

71 

35 

32 

29 

25 

21 

16 

9 

0 

—17 

71 

72 

37 

33 

31 

27 

-  23 

18 

13 

+  5 

—  6 

—39 

72 

73 

38 

35 

32 

29 

25 

21 

16 

10 

0 

—16 

73 

74 

40 

37 

34 

31 

28 

24 

19 

13 

+  6 

—  6 

—37 

74 

75 

42 

39 

36 

32 

30 

26 

22 

16 

10 

0 

—16 

75 

76 

43 

41 

38 

34 

32 

28 

24 

20 

14 

+  6 

—  G 

—34 

76 

77 

45 

42 

40 

36 

33 

30 

26 

22 

17 

11 

+  1 

—14 

77 

78 

47 

44 

41 

38 

35 

32 

28 

24 

20 

14 

7 

—  4 

—30 

78 

79 

48 

46 

43 

40 

37 

34 

31 

27 

23 

18 

11 

+  2 

—13 

79 

80 

50 

47 

45 

42 

39 

36 

32 

29 

25 

21 

15 

8 

—  3 

80 

t. 

18°.0 

19°.0 

20°.0 

21°.  0 

22°.0 

23°  0 

24°.0 

25°.0 

26°.  0 

27°.  0 

28°.0 

29°.0 

30°.  0 

t. 

77 


TABLE  I. — Temperature  of  the  deiv  point,  in  degrees  Fahrenheit. 


J 

Difference  between  the  dry  and  wet  thermometers  (tr-tf). 

I 

b 

a 

1°.0 

2°.0 

3°.0 

4°.0 

5°.0 

6°.0 

7°.0 

8°.0 

9°.0 

10°.  0 

11°.0 

12°.0 

1 

80 

79 

77 

76 

74 

73 

72 

70 

68 

67 

65 

63 

62 

80 

81 

80 

78 

77 

75 

74 

73 

71 

70 

68 

66 

6.5 

63 

81 

82 

81 

79 

78 

77 

75 

74 

72 

71 

69 

68 

66 

64 

82 

83 

82 

80 

79 

78 

76 

75 

73 

72 

70 

69 

67 

65 

83 

84 

83 

81 

80 

79 

77 

76 

74 

73 

71 

70 

68 

67 

84 

85 

84 

82 

81 

80 

78 

77 

75 

74 

72 

71 

69 

68 

85 

86 

85 

83 

82 

81 

79 

78 

76 

75 

73 

72 

71 

69 

86 

87 

86 

84 

83 

82 

80 

79 

78 

76 

74 

73 

72 

70 

87 

88 

87 

85 

84 

83 

81 

80 

79 

77 

75 

74 

73 

71 

88 

89 

88 

86 

85 

84 

82 

81 

80 

78 

76 

76 

74 

72 

89 

90 

89 

87 

86 

85 

84 

82 

81 

79 

78 

77 

75 

74 

90 

91 

90 

88 

87 

86 

85 

83 

82 

80 

79 

78 

76 

75 

91 

92 

91 

89 

88 

87 

86 

84 

83 

82 

80 

79 

77 

76 

92 

93 

92 

91 

89 

88 

87 

85 

84 

83 

81 

80 

78 

77 

93 

94 

93 

92 

90 

89 

88 

86 

85 

84 

82 

81 

80 

78 

94 

95 

94 

93 

91 

90 

89 

87 

86 

85 

83 

82 

81 

79 

95 

96 

95 

94 

92 

91 

90 

88 

87 

86 

84 

83 

82 

80 

96 

97 

96 

95 

93 

92 

91 

90 

88 

87 

86 

84 

83 

81 

97 

98 

97 

96 

94 

93 

92 

91 

89 

88 

87 

85 

84 

83 

98 

99 

98 

97 

95 

94 

93 

92 

90 

89 

88 

86 

85 

84 

99 

100 

99 

98 

96 

95 

94 

93 

91 

90 

89 

87 

86 

85 

100 

101 

100 

99 

97 

96 

95 

94 

92 

91 

90 

88 

87 

86 

101 

102 

101 

100 

98 

97 

96 

95 

93 

92 

91 

90 

88 

87 

102 

103 

102 

101 

99 

98 

97 

96 

94 

93 

92 

91 

89 

88 

103 

104 

103 

102 

100 

99 

98 

97 

96 

94 

93 

92 

90 

89 

104 

105 

104 

103 

101 

100 

99 

98 

97 

95 

94 

93 

91 

•90 

105 

106 

105 

104 

102 

101 

100 

99 

98 

96 

95 

94 

93 

91 

106 

107 

106 

105 

103 

102 

101 

100 

99 

97 

96 

95 

94 

92 

107 

108 

107 

106 

104 

103 

102 

101 

100 

98 

97 

96 

95 

93 

108 

109 

108 

107 

105 

104 

103 

102 

101 

99 

98 

97 

96 

94 

109 

110 

109 

108 

107 

105 

104 

.103 

102 

101 

99 

98 

97 

96 

110 

111 

110 

109 

108 

106 

105 

104 

103 

102 

100 

99 

98 

97 

111 

112 

111 

110   109 

107 

106 

105 

104 

103 

101 

100 

99 

98 

112 

113 

112 

111 

110 

108 

107 

106 

105 

104 

102 

101 

100 

99 

113 

114 

113 

112 

111 

109 

108 

107 

106 

105 

103 

102 

101 

100 

114 

115 

114 

113 

112 

110 

109 

108 

107 

106 

105 

103 

102 

101 

115 

116 

115 

114 

113 

111 

110 

109 

108 

107 

106 

104 

103 

102 

116 

117 

116 

115 

114 

112 

111 

110 

109 

108 

107 

105 

104 

103 

117 

118 

117 

116 

115 

113 

112 

111 

110 

109 

108 

106 

105 

104 

118 

119 

118 

117 

116 

114 

113 

112 

111 

110 

109 

107 

106 

105 

119 

120 

119 

118 

117 

115 

114 

113 

112 

111 

110 

108 

107 

106 

120 

121 

120   119 

118 

117 

115 

114 

113 

112 

111 

110 

108 

107 

121 

122 

121 

120 

119 

118 

116 

115 

114 

113 

112 

111 

109 

108 

122 

123 

122 

121 

120 

119 

117 

116 

115 

114 

113 

112 

110 

109 

123 

124 

123 

122 

121 

120 

118 

117 

116 

115 

114 

113 

111 

110 

124 

125 

124 

123 

122 

121 

119 

118 

117 

116 

115 

114 

112 

111 

1-25 

126 

125 

124 

123 

122 

120 

119 

118 

117 

116 

115 

114 

112 

126 

127 

r/6 

125 

124 

123 

121 

120 

119 

118 

117 

116 

115 

113 

127 

128 

127 

126 

125 

124 

122 

121 

120 

119 

118 

117 

116 

114 

128 

129 

128 

127 

126 

125 

123 

122 

121 

120 

119 

118 

117 

115 

129 

130 

129 

128 

127 

126 

124 

123 

122 

121 

120 

119 

118 

117 

130 

131 

130 

129 

128 

127 

125 

124 

123 

122 

121 

120 

119 

118 

131 

132 

131 

130 

129 

128 

126 

125 

124 

123 

122 

121 

120 

119 

132 

133 

132 

131 

130 

129 

128 

126  '  125   124 

123 

122 

121 

120 

133 

134 

133 

132 

131 

130 

129 

127 

126 

125 

124 

123 

122 

121 

134 

135 

134 

133 

132 

131 

130 

128 

127 

126 

125 

124 

123 

122 

135 

136 

135 

134 

133 

132 

131 

129 

128 

127 

126 

125 

124 

123 

136 

137 

136 

135 

134 

133 

132 

130 

129 

128 

127 

126 

125 

124 

137 

138 

137 

136 

135 

134 

133 

131 

J30 

129 

128 

127 

126 

125 

138 

139 

138 

137 

136 

135 

134 

132 

131 

130 

129 

128 

127 

126 

139 

140 

139 

138 

137 

136 

135 

133 

132 

131 

130 

129 

128 

127 

140 

t. 

1°.0 

2°.0 

3°.0 

4°.0 

5°.0 

6°.0 

7°.0 

8°.0 

9°.0 

10°.0 

11°.  0 

12.°0 

I. 

78 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


t  . 

(Dryther.) 

Difference  between  the  dry  and  wet  thermometers  (<  —  t'). 

? 

~£ 

I 

12°.0 

13°.0 

14°.0 

15°.0 

16°.  0 

17°.0 

18°.0 

19°.0 

20°.0 

21°.0 

22°.0 

23°.0 

24°.0 

80 

62 

60 

58 

56 

54 

52 

50 

47 

45 

42 

39 

36 

32 

80 

81 

63 

61 

59 

57 

55 

53 

51 

49 

47 

44 

41 

38 

35 

81 

82 

64 

62 

61 

59 

57 

55 

53 

50 

48 

45 

43 

40 

37 

82 

83 

65 

64 

62 

60 

58 

56 

54 

52 

50 

47 

44 

42 

39 

83 

84 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

46 

43 

41 

84 

85 

68 

66 

64 

62 

61 

59 

57 

55 

53 

50 

48 

45 

42 

85 

86 

69 

67 

66 

64 

62 

60 

58 

56 

54 

52 

49 

47 

44 

86 

87 

70 

68 

67 

65 

63 

6o 

59 

57 

55 

53 

51 

48 

46 

87 

88 

71 

70 

68 

66 

64 

63 

61 

59 

57 

55 

53 

50 

48 

88 

89 

72 

71 

69 

67 

66 

64 

62 

60 

58 

56 

54 

52 

49 

89 

90 

74 

72 

70 

69 

67 

65 

63 

62 

60 

58 

56 

53 

51 

90 

91 

75 

73 

72 

70 

68 

67 

65 

63 

61 

59 

57 

55 

53 

91 

92 

76 

74 

73 

71 

69 

68 

66 

64 

62 

60 

58 

56 

54 

92 

93 

77 

75 

74 

72 

71 

69 

67 

66 

64 

62 

60 

58 

66 

93 

94 

78 

77 

75 

73 

72 

70 

69 

67 

65 

63 

61 

59 

57 

94 

95 

79 

78 

76 

75 

73 

71 

70 

68 

66 

64 

63 

61 

59 

95 

96 

80 

79 

77 

76 

74 

73 

71 

69 

68 

66 

64 

62 

60 

96 

97 

81 

80 

78 

77 

75 

74 

72 

71 

69 

67 

65 

63 

61 

97 

98 

83 

81 

80 

78 

77 

75 

73 

72 

70 

68 

67 

65 

63 

98 

99 

84 

82 

81 

79 

78 

76 

75 

73 

71 

70 

68 

66 

64 

99 

100 

85 

83 

82 

80 

79 

77 

76 

74 

73 

71 

69 

67 

66 

100 

101 

86 

84 

83 

82 

80 

79 

77 

75 

74 

72 

71 

69 

67 

101 

102 

87 

85 

84 

83 

81 

80 

78 

77 

75 

73 

72 

70 

68 

102 

103 

88 

87 

85 

84 

82 

81 

79 

78 

76 

75 

73 

71 

70 

103 

104 

89 

88 

86 

85 

83 

82 

81 

79 

78 

76 

74 

73 

71 

104 

105 

90 

89 

87 

86 

85 

83 

82 

80 

79 

77 

76 

74 

72 

105 

106 

91 

90 

89 

87 

86 

84 

83 

81 

80 

78 

77 

75 

74 

106 

107 

92 

91 

90 

88 

87 

85 

84 

83 

81 

80 

78 

76 

75 

107 

108 

93 

92 

91 

89 

88 

87 

85 

84 

82 

81 

79 

78 

76 

108 

109 

94 

93 

92 

90 

89 

88 

86 

85 

83 

«2 

80 

79 

77 

109 

110 

96 

94 

93 

92 

90 

89 

87 

86 

85 

83 

82 

80 

79 

110 

111 

97 

95 

94 

93 

91 

90 

89 

87 

86 

84 

83 

81 

80 

111 

112 

98 

96 

95 

94 

92 

91 

90 

88 

87 

85 

84 

83 

81 

112 

113 

99 

97 

96 

95 

93 

92 

91 

89 

88 

87 

85 

84 

82 

113 

114 

100 

99 

97 

96 

95 

93 

92 

91 

89 

88 

86 

85 

83 

114 

115 

101 

100 

98 

97 

96 

94 

93 

92 

90 

89 

88 

86 

85 

115 

116 

102 

101 

99 

98 

97 

95 

94 

93 

91 

90 

89 

87 

86 

116 

117 

103 

102 

100 

99 

98 

96 

95 

94 

93 

91 

90 

88 

87 

117 

118 

104 

103 

101 

100 

99 

97 

96 

95 

94 

92 

91 

90 

88 

118 

119 

105 

104 

103 

101 

100 

99 

97 

96 

95 

93 

92 

91 

89 

119 

120 

106 

105 

104 

102 

101 

100 

99 

97 

96 

95 

93 

92 

90 

120 

121 

107 

106 

105 

103 

102 

101 

100 

98 

97 

96 

94 

93 

92 

121 

122 

108 

107 

106 

105 

103 

102 

101 

99 

98 

97 

96 

94 

93 

122 

123 

109 

108 

107 

106 

104 

103 

102 

101 

99 

98 

97 

95 

94 

123 

124 

110 

109 

108 

107 

105 

104 

103 

102 

100 

99 

98 

96 

95 

124 

125 

111 

110 

109 

108 

106 

105 

104 

103 

101 

100 

99 

98 

96 

125 

126 

112 

111 

110 

109 

108 

106 

105 

104 

103 

101 

100 

99 

97 

126 

127 

113 

112 

111 

110 

109 

107 

106 

105 

104 

102 

101 

100 

98 

127 

128 

114 

113 

112 

111 

110 

108 

107 

106 

105 

103 

102 

101 

100 

128 

129 

115 

114 

113 

112 

111 

110 

108 

107 

106 

105 

103 

102 

101 

129 

130 

117 

115 

114 

113 

112 

111 

109 

108 

107 

106 

104 

103 

102 

130 

131 

118 

116 

115 

114 

113 

112 

110 

109 

108 

107 

106 

104 

103 

131 

132 

119 

117 

116 

115 

114 

113 

112 

110 

109 

108 

107 

105 

104 

132 

133 

120 

118 

117 

116 

115 

114 

113 

111 

110 

109 

108 

106 

105 

133 

134 

121 

120 

118 

117 

116 

115 

114 

112 

111 

110 

109 

108 

106 

134 

135 

122 

121 

119 

118 

117 

116 

115 

113 

112 

111 

110 

109 

107 

135 

136 

123 

122 

120 

119 

118 

117 

116 

115 

113 

112 

111 

110 

108 

136 

137 

124 

123 

121 

120 

119 

118 

117 

116 

114 

113 

112 

111 

110 

137 

138 

125 

124 

122 

121 

120 

119 

118 

117 

115 

114 

113 

112 

111 

138 

139 

126 

125 

124 

122 

121 

120 

119 

118 

117 

115 

114 

113 

112 

139 

140 

127 

126 

125 

123 

122 

121 

120 

119 

118 

116 

115 

114 

113 

140 

t. 

12°.  0 

13°.0 

14°.0 

15°.  0 

16°.0 

17°.0 

18°.0 

19°.0 

20°.0 

21°.0 

22°.0 

23°.0 

24°.0 

i 

79 


TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


I 

Difference  between  the  dry  and  wet  thermometers  (  t  —  <'). 

I 

1 

24°.  0 

25°.0 

26°.0 

27°.  0 

28°.0 

29°.0 

30°.0 

31°.0 

32°.  0 

33°.0 

34°.0 

35°.0 

36°.  0 

"I 

80 

32 

29 

25 

21 

15 

8 

—3 

—27 

80 

81 

35 

31 

28 

24 

18 

12 

+3 

—11 

81 

82 

37 

33 

30 

26 

'  22 

16 

9 

—  2 

-24 

82 

83 

39 

35 

32 

28 

24 

19 

13 

+  5 

—  9 

83 

84 

41 

37 

34 

30 

27 

22 

17 

10 

0 

-20 

84 

85 

42 

39 

36 

32 

29 

25 

20 

14 

+  6 

-  7 

—64 

85 

86 

44 

41 

38 

35 

31 

28 

23 

18 

11 

+  1 

—17 

86 

87 

46 

43 

40 

37 

33 

30 

26 

21 

15 

7 

—  5 

—38 

87 

88 

48 

45 

42 

39 

35 

32 

28 

24 

19 

12 

+  3 

—13 

88 

89 

49 

47 

44 

41 

38 

34 

31 

27 

22 

16 

9 

2 

—28 

89 

90 

51 

48 

46 

43 

40 

36 

32 

29 

25 

20 

13 

+  * 

—10 

90 

91 

53 

50 

47 

45 

42 

38 

35 

32 

28 

23 

18 

10 

0 

91 

92 

54 

52 

49 

46 

44 

41 

37 

34 

30 

26 

21 

15 

+  7 

92 

93 

56 

63 

51 

48 

46 

43 

39 

36 

32 

29 

24 

19 

12 

93 

94 

57 

55 

53 

50 

47 

45 

42 

38 

35 

31 

27 

22 

16 

94 

95 

59 

56 

54 

52 

49 

46 

44 

40 

37 

33 

30 

25 

20 

95 

96 

60 

58 

56 

53 

51 

48 

46 

43 

39 

36 

32 

28 

24 

96 

97 

61 

59 

57 

55 

53 

60 

47 

45 

41 

38 

34 

31 

26 

97 

98 

63 

61 

59 

57 

54 

52 

49 

47 

44 

40 

37 

33 

29 

98 

99 

64 

02 

60 

58 

56 

54 

51 

48 

46 

43 

39 

35 

32 

99 

100 

66 

64 

62 

60 

57 

55 

53 

50 

48 

46 

41 

38 

34 

100 

101 

67 

65 

63 

61 

59 

57 

54 

52 

49 

4V 

44 

40 

37 

101 

102 

68 

66 

65 

63 

61 

58 

56 

54 

51 

49 

46 

43 

39 

102 

103 

70 

68 

66 

64 

62 

60 

58 

55 

53 

50 

48 

45 

41 

103 

104 

71 

69 

67 

65 

63 

61 

59 

57 

55 

52 

50 

47 

44 

104 

105 

72 

"(0 

69 

67 

65 

63 

61 

59 

56 

54 

52 

49 

'  46 

105 

106 

74 

V2 

70 

68 

66 

64 

62 

60 

58 

56 

53 

51 

48 

106 

107 

75 

73 

71 

VO 

68 

66 

64 

62 

60 

57 

55 

52 

50 

107 

108 

76 

74 

73 

Yl 

69 

67 

65 

63 

61 

59 

57 

54 

52 

108 

109 

77 

76 

74 

72 

71 

69 

67 

65 

63 

61 

58 

56 

54 

109 

110 

79 

77 

75 

74 

72 

70 

68 

66 

64 

62 

60 

58 

55 

110 

II1 

80 

73 

77 

75 

73 

71 

70 

68 

66 

64 

62 

59 

57 

111 

112 

81 

79 

78 

76 

74 

73 

71 

69 

67 

65 

63 

61 

59 

112 

113 

82 

81 

79 

77 

76 

74 

72 

71 

69 

67 

65 

63 

60 

113 

114 

83 

82 

80 

79 

77 

75 

74 

72 

70 

68 

66 

64 

62 

114 

115 

85 

83 

82 

80 

78 

77 

75 

73 

72 

70 

68 

66. 

64 

115 

116 

86 

84 

83 

81 

80 

78 

76 

75 

73 

71 

69 

67 

65 

116 

117 

87 

86 

84 

82 

81 

79 

78 

76 

74 

72 

71 

69 

67 

117 

118 

88 

87 

85 

84 

82 

81 

79 

77 

76 

74 

72 

70 

68 

118 

119 

89 

88 

86 

85 

83 

82 

80 

79 

77 

75 

74 

72 

70 

•  119 

120 

90 

89 

88 

86 

85 

83 

82 

80 

78 

77 

75 

73 

71 

120 

121 

92 

90 

89 

87 

86 

84 

83 

81 

80 

78 

76 

74 

73 

121 

122 

93 

91 

90 

89 

87 

86 

84 

83 

81 

79 

78 

76 

74 

122 

123 

94 

93 

91 

90 

88 

87 

85 

84 

82 

81 

79 

77 

75 

123 

124 

96 

94 

92 

91 

89 

88 

87 

85 

83 

82 

80 

79 

77 

124 

125 

96 

95 

93 

92 

91 

89 

88 

86 

85 

83 

82 

80 

78 

125 

126 

97 

96 

95 

93 

92 

90 

89 

87 

86 

84 

83 

81 

80 

126 

127 

98 

97 

96 

94 

93 

92 

90 

89 

87 

86 

84 

83 

81 

127 

128 

100 

98 

97 

96 

94 

93 

91 

90 

88 

87 

85 

84 

82 

128 

129 

101 

99 

98 

97 

95 

94 

93 

91 

90 

88 

87 

85 

84 

129 

130 
131 
132 
133 
134 

102 

103 
104 
105 
106 

101 
102 
103 
104 
105 

99 
100 
101 
103 
104 

98 
99 
100 
101 
102 

97 
98 
99 
100 
101 

95 
96 
97 
99 
100 

94 
95 
96 
97 
98 

92 
94 
95 
96 

97 

91 
92 
93 
94 
96 

89 
91 
92 
93 
94 

88 
89 
90 
92 
93 

86 
88 
89 
90 
91 

85 
86 
87 
89 
90 

130 
131 
132 
133 
134 

135 
136 
137 
138 
139 

107 
108 
110 
111 
112 

106 
107 
108 
109 
110 

105 
106 
107 
108 
109 

104 
105 
106 
107 
108 

102 
103 
104 
106 
107 

101 
102 
103 
104 
105 

100 
101 
102 
103 
104 

98 
99 
101 
102 
103 

97 
98 
99 
100 
101 

95 
97 
98 
99 
100 

94 
95 
96 

98 
99 

93 
94 
95 
96 
97 

91 
92 
94 
95 
96 

135 
136 
137 
138 
139 

140 

113 

112 

110 

109 

108 

107 

105 

104 

103 

101 

100 

99 

97 

140 

t. 

24°.  0 

25°  .0 

26°.  0 

27°.0 

28°.0 

29°.0 

30°.0 

31°.0 

32°.0 

33°.  0 

34°.0 

35°.0 

36°.0 

t. 

TABLE  I. — Temperature  of  the  dew  point,  in  degrees  Fahrenheit. 


1 

Difference  between  the  dry  and  wet  thermometers,  (<—<').  - 

? 

I 

36°.0 

37  -.0 

38°.0 

39°.0 

40°.0 

41°.  0 

42°.0 

43°.0 

44°.  0 

45°0 

46°.  0 

47°.  0 

48°.0 

b 

89 

—28 

89 

90 

—10 

• 

90 

91 

0 

-22 

91 

92 

+  7 

—  7 

—16 

92 

93 

12 

+  2 

—  4 

93 

94 

16 

8 

+  4 

—37 

94 

95 

20 

13 

10 

—12 

95 

96 

24 

15 

15 

-  1 

—25 

96 

97 

26 

21 

19 

+  7 

—  8 

97 

98 

29 

25 

23 

12 

+  2 

—18 

98 

99 

32 

28 

26 

17 

9 

—  4 

-42 

99 

100 

34 

30 

29 

21 

14 

+  5 

—12 

100 

101 

37 

32 

32 

24 

18 

11 

0 

—25 

101 

102 

39 

35 

34 

27 

22 

16 

+  7 

—  7 

102 

103 

41 

38 

34 

30 

26 

20 

13 

+  3 

—16 

103 

104 

44 

40 

37 

32 

29 

24 

18 

10 

—  2 

-38 

104 

105 

46 

43 

39 

35 

31 

27 

22 

15 

+  6 

—10 

105 

106 

48 

45 

42 

38 

34 

30 

25 

20 

12 

+  1 

-22 

106 

107 

50 

47 

44 

40 

37 

32 

28 

24 

17 

9 

—  5 

107 

108  ' 

52 

49 

46 

43 

39 

35 

31 

27 

21 

14 

+  5 

-13 

108 

109 

54 

51 

48 

43 

42 

38 

34 

30 

25 

-  19 

12 

0 

—28 

109 

110 

55 

53 

50 

47 

44 

41 

37 

33 

28 

23 

17 

+  8 

—  7 

110 

111 

57 

54 

52 

49 

46 

43 

39 

35 

31 

27 

21 

14 

+  4 

111 

112 

59 

56 

54 

51 

48 

45 

42 

38 

34 

30 

25 

19 

11 

112 

113 

60 

58 

56 

53 

50 

47 

44 

41 

37 

32 

28 

23 

16 

113 

114 

62 

60 

58 

55 

52 

50 

47 

43 

40 

35 

31 

27 

21 

114 

115 

64 

62 

59 

57 

54 

52 

49 

46 

42 

38 

34 

30 

25 

115 

116 

65 

63 

61 

59 

56 

54 

51 

43 

45 

41 

37 

33 

28 

116 

117 

67 

65 

63 

60 

58 

56 

53 

50 

47 

44 

40 

36 

32 

117 

118 

68 

66 

64 

62 

60 

57 

55 

52 

49 

46 

43 

39 

31 

118 

119 

70 

68 

66 

64 

61 

59 

57 

54 

51 

48 

45 

42 

37 

119 

120 

71 

69 

67 

65 

63 

61 

59 

56 

53 

50 

47 

44 

40 

120 

121 

73 

71 

69 

67 

65 

63 

60 

58 

55 

52 

50 

47 

43 

121 

122 

74 

72 

70 

68 

66 

64 

62 

60 

57 

54 

52 

49 

46 

122 

123 

75 

74 

7-2 

70 

68 

66 

64 

61 

59 

56 

54 

51 

48 

123 

124 

77 

75 

73 

71 

69 

67 

65 

63 

61 

58 

56 

53 

50 

124 

125 

78 

76 

75 

73 

71 

69 

67 

65 

63 

60 

58 

55 

52 

125 

126 

80 

78 

76 

74 

73 

71 

69 

67 

64 

62 

60 

57 

54 

126 

127 

81 

79 

78 

76 

74 

72 

70 

68 

66 

64 

62 

59 

56 

127 

128 

82 

81 

79 

77 

75 

74 

72 

70 

68 

65 

63 

61 

58 

128 

129 

84 

82 

80 

79 

77 

75 

73 

71 

69 

67 

65 

63 

60 

129 

130 

85 

83 

82 

80 

78 

76 

75 

73 

71 

69 

67 

64 

62 

130 

131 

86 

85 

83 

81 

80 

78 

76 

74 

72 

70 

68 

66 

64 

131 

132 

87 

86 

84 

83 

81 

79 

78 

76 

74 

72 

70 

68 

66 

132 

133 

89 

87 

86 

84 

82 

81 

79 

77 

75 

73 

71 

69 

67 

133 

134 

90 

88 

87 

85 

84 

82 

80 

79 

77 

75 

73 

71 

69 

134 

135 

91 

90 

88 

87 

85 

83 

82 

£0 

78 

76 

75 

73 

71 

135 

136 

92 

91 

89 

88 

86 

85 

83 

81 

80 

78 

76 

74 

72 

136 

137 

94 

92 

91 

89 

88 

86 

84 

83 

81 

79 

78 

76 

74 

137 

138 

95 

93 

92 

90 

89 

87 

86 

84 

82 

81 

79 

77 

75 

138 

139 

96 

95 

93 

92 

90 

89 

87 

85 

84 

82 

80 

79 

77 

139 

140 

97 

96 

94 

93 

91 

90 

88 

87 

85 

83 

82 

80 

78 

140 

t. 

36°.0 

37°.  0 

38°.0 

39°.0 

40°.  0 

41°.0 

42°.0 

43°.  0 

44°.  0 

45°  .0 

46°.0 

47°.  0 

48°.0 

I. 

81 


TABLE  II. — Relative  humidity,  per  cent. 


•K.  •£ 

Difference  between  the  dry  and  wet  thermometers  (t—f). 

_i 

b 

0°.2 

0°.4 

0°.6 

0°.8 

10.0 

10.. 

10.4 

10.6 

1».8 

2°.0 

2°.  2 

2°.  4 

2°.  6 

"i 

—40 

46 

~ 

(<-<') 

—40 

—39 

49 

JT 

—39 

—38 

51 

—38 

—37 

54 

b 

0°.  1 

0°.2 

—37 

—36 

56 

— 

—36 

—35 

59 

—50 

51 

—  35 

—34 

61 

22 

—49 

54 

—34 

—33 

63 

25 

—48 

57 

—  33   Rela- 

—32 
—31 

65 
67 

30 
34 

—47 

—46 

60 

62 

-3?  tivehu- 
—  31  midity. 

—30 

69 

38 

—45 

65 

—30 

—29 

71 

42 

—44 

67 

33 

—29 

-28 

72 

45 

17 

—43 

69 

37 

—28 

—27 

74 

48 

22 

—42 

71 

40 

—27 

-26 

76 

51 

26 

—41 

72 

44 

—26 

-25 

77 

53 

31 

—40 

73 

46 

—25 

-24 

78 

56 

34 

12 

—39 

74 

49 

—24 

-23 

79 

58 

37 

16 

—38 

76 

51 

—23 

—22 

80 

60 

40 

20 

—37 

77 

54 

—22 

—21 

81 

62 

44 

25 

—36 

78 

56 

—21 

—20 

82 

64 

47 

29 

11 

—35 

79 

59 

—20 

—19 

83 

66 

49 

33 

16 

—34 

80 

61 

—19 

—18 

84 

68 

52 

36 

20 

-33 

81 

63 

—18 

—17 

85 

70 

54 

39 

24 

9 

—32 

83 

65 

-17 

—16 

86 

71 

57 

43 

28 

14 

—31 

84 

67 

—16 

—15 

1  A_ 

86 

73 

59 

46 

AQ 

32 

Q£» 

19 
23 

10 

—30 

85 

69 

—15 

—13 

88 

76 

63 

T:O 

51 

OD 

39 

27 

15 

—13 

—12 

88 

77 

65 

53 

42 

30 

19 

7 

—12 

—11 

89 

78 

67 

56 

45 

34 

23 

12 

—11 

—10 

90 

79 

68 

58 

48 

37 

26 

16 

5 

—10 

—  9 

90 

80 

70 

60 

50 

40 

30 

20 

10 

—  9 

—  8 

90 

81 

71 

62 

52 

'43 

33 

24 

14 

5 

—  8 

—  7 

91 

82 

73 

63 

54 

45 

36 

27 

18 

9 

—  7 

—  6 

91 

83 

74 

65 

56 

48 

39 

31 

22 

13 

5 

—  6 

—  5 

92 

83 

75 

67 

58 

50 

42 

34 

25 

17 

9 

—  5 

—  4 

92 

84 

76 

68 

60 

52 

45 

37 

29 

21 

13 

5 

—  4 

3 

92 

85 

77 

70 

62 

65 

47 

40 

32 

25 

17 

10 

—  3 

—  2 

93 

86 

78 

71 

64 

57 

50 

42 

35 

28 

21 

14 

7 

—  2 

—  1 

93 

86 

79 

72 

66 

59 

52 

45 

38 

31 

25 

18 

11 

—  1 

0 

93 

87 

80 

74 

67 

61 

54 

48 

41 

35 

28 

22 

15 

0 

+  1 

94 

87 

81 

75 

69 

63 

56 

50 

44 

38 

32 

25 

19 

+  1 

2 

94 

88 

82 

76 

70 

64 

58 

52 

46 

40 

35 

29 

23 

2 

3 

94 

88 

83 

77 

71 

66 

60 

54 

49 

43 

37 

32 

26 

3 

4 

94 

89 

83 

78 

73 

67 

62 

56 

51 

45 

40 

34 

29 

4 

5 

95 

89 

84 

79 

74 

68 

63 

58 

53 

48 

42 

3.7 

32 

5 

6 

95 

90 

85 

80 

75 

70 

65 

60 

54 

50 

44 

39 

34 

6 

7 

95 

90 

85 

80 

76 

71 

66 

61 

56 

51 

47 

42 

37 

7 

8 

95 

91 

86 

81 

76 

72 

67 

62 

58 

53 

49 

44 

39 

8 

9 

96 

91 

86 

82 

77 

73 

68 

64 

59 

55 

51 

46 

42 

9 

10 

96 

91 

87 

83 

78 

74 

69 

65 

61 

57 

52 

48 

44 

10 

It 

96 

92 

87 

83 

79 

75 

71 

66 

62 

58 

54 

50 

46 

11 

12 

90 

92 

88 

84 

80 

76 

72 

68 

64 

60 

56 

52 

48 

12 

13 

96 

92 

88 

84 

81 

77 

73 

69 

65 

61 

58 

54 

50 

13 

14 

96 

93 

89 

85 

81 

78 

74 

70 

67 

63 

59 

56 

52 

14 

15 

96 

93 

89 

86 

82 

79 

75 

71 

68 

64 

61 

57 

54 

15 

16 

97 

93 

90 

8G 

83 

79 

76 

73 

69 

66 

62 

59 

56 

16 

17 

97 

93 

90 

87 

83 

80 

77 

74 

70 

67 

64 

60 

57 

17 

18 

97 

94 

90 

87 

84 

81 

78 

74 

71 

68 

65 

62 

59 

18 

19 

97 

94 

91 

88 

84 

81 

78 

75 

72 

69 

66 

63 

60 

19 

20 

97 

94 

91 

88 

85 

82 

79 

76 

73 

70 

67 

64 

61 

20 

t. 

0°.2 

0°.4 

0°.6 

0°.8 

1°.  0 

,».2 

10.4 

P.  6 

,0.8 

2°.0 

2°.2 

2°.  4 

2°.  6 

t. 

82 


TABLE  II. — Relative  humidity,  per  cent. 


I 

Difference  between  the  dry  and  wet  thermometers  (t  —  <')• 

£ 

p. 

2°.  6 

2°.  8 

3°.0 

3°.  2 

3°.  4 

3°.  6 

3°.  8 

4°.0 

4°.  2 

4°.  4 

1 

—  2 

7 

—  2 

—  1 

11 

4 

—  1 

o 

15 

9 

0 

+  1 

19 

13 

7 

-f  1 

2 

23 

17 

11 

5 

2 

3 

26 

20 

15 

9 

4 

3 

4 

29 

24 

18 

13 

8 

2 

4 

5 

32 

27 

22 

16 

11 

6 

5 

6 

34 

29 

25 

20 

15 

10 

5 

6 

7 

37 

32 

28 

23 

18 

13 

9 

4 

7 

8 

39 

35 

30 

26 

21 

17 

12 

8 

3 

8 

9 

42 

37 

33 

28 

24 

20 

15 

11 

7 

2 

9 

10 

44 

40 

35 

31 

27 

23 

19 

14 

10 

6 

10 

11 

46 

42 

38 

34 

30 

26 

22 

18 

14 

10 

11 

12 

48 

44 

40 

36 

32 

28 

25 

21 

17 

13 

12 

13 

50 

46 

42 

39 

35 

31 

27 

24 

20 

16 

13 

14 

52 

48 

45 

41 

37 

34 

30 

27 

23 

19 

14 

15 

54 

50 

47 

43 

40 

36 

33 

29 

26 

23 

15 

16 

56 

52 

49 

46 

42 

39 

36 

32 

29 

25 

16 

17 

57 

54 

51 

48 

44 

41 

38 

35 

31 

28 

17 

18 

59 

56 

53 

49 

46 

43 

40 

37 

34 

31 

18 

19 

60 

57 

54 

51 

48 

45 

42 

39 

36 

33 

19 

20 

61 

58 

56 

53 

50 

47 

44 

41 

38 

'35 

20 

t. 

2°.  6 

2°.8 

3°.0 

3°.  2 

3°.  4 

3°.  6 

3°.  8 

4°.0 

4°.  2 

4°.  4 

t. 

t. 

4°.  6 

4°.  8 

5°.0 

5°.  2 

5°.  4 

5°.  6 

5°.  8 

6°.0 

6°.  2 

6°.  4 

t. 

8 

8 

9 

9 

10 

2 

10 

11 

6 

2 

11 

12 

9 

5 

2 

12 

13 

13 

9 

5 

2 

13 

14 

16 

12 

9 

5 

2 

14 

15 

19 

16 

12 

9 

5 

2 

15 

16 

22 

19 

16 

12 

9 

6 

2 

16 

17 

25 

22 

19 

16 

12 

9 

6 

3 

17 

18 

28 

25 

22 

19 

16 

13 

9 

6 

3 

18 

19 

30 

27 

24 

21 

19 

16 

13 

10 

7 

4 

19 

20 

33 

30 

27 

24 

21 

19 

16 

13 

10 

7 

20 

t. 

4°.  6 

4°.  8 

5°.0 

5°.  2 

5°.  4 

5°.  6 

5°.  8 

6°.0 

6^.2 

6°.  4 

t. 

83 


TABLE  II. — Relative  humidity,  per  cent. 


X5 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

1 

I 

0°.5 

10.0 

10.5 

2°.0 

2°.  5 

3°.0 

3°.  5 

4°.0 

4°.  5 

5°.0 

5°.  5 

6°.0 

1 

20 

92 

85 

77 

70 

63 

56 

48 

41 

34 

27 

20 

13 

20 

21 

93 

85 

78 

71 

64 

57 

50 

43 

36 

29 

23 

16 

21 

22 

93 

86 

79 

72 

65 

58 

51 

45 

38 

32 

25 

19 

22 

23 

93 

86 

80 

73 

66 

60 

53 

46 

40 

34 

27 

21 

23 

24 

93 

87 

80 

74 

67 

61 

54 

48 

42 

36 

30 

24 

24 

25 

94 

87 

81 

74 

68 

62 

56 

50 

44 

38 

32 

26 

25 

26 

94 

88 

81 

75 

69 

63 

57 

51 

45 

40 

34 

28 

26 

27 

94 

88 

82 

76 

70 

64 

59 

53 

47 

42 

36 

30 

27 

28 

94 

88 

82 

77 

71 

65 

GO 

54 

49 

43 

38 

33 

28 

29 

94 

89 

83 

77 

72 

66 

61 

56 

50 

45 

40 

35 

29 

30 

94 

89 

84 

78 

73 

67 

62 

57 

52 

47 

41 

36 

30 

31 

95 

89 

84 

79 

74 

68 

63 

58 

53 

48 

43 

38 

31 

32 

95 

90 

84 

79 

74 

69 

64 

59 

54 

50 

45 

40 

32 

33 

95 

90 

85 

80 

75 

70 

65 

60 

56 

51 

47 

42 

33 

34 

95 

91 

86 

81 

75 

72 

67 

62 

57 

53 

48 

44 

34 

35 

95 

91 

86 

82 

76 

73 

69 

65 

59 

54 

50 

45 

35 

36 

96 

91 

86 

82 

77 

73 

70 

66 

61 

56 

51 

47 

36 

37 

96 

91 

87 

82 

78 

74 

70 

66 

62 

57 

52 

48 

37 

38 

96 

92 

87 

83 

79 

75 

71 

67 

63 

58 

54 

50 

38 

39 

96 

9'^ 

88 

83 

79 

75 

72 

68 

63 

59 

55 

52 

39 

40 

96 

92 

88 

84 

80 

76 

72 

68 

64 

60 

56 

53 

40 

41 

96 

92 

88 

84 

80 

76 

72 

69 

65 

61 

57 

54 

41 

42 

96 

92 

88 

84 

81 

77 

73 

69 

65 

62 

58 

55 

42 

43 

96 

92 

88 

85 

81 

77 

74 

70 

66 

63 

59 

56 

43 

44 

96 

92 

88 

85 

81 

78 

74 

70 

67 

63 

60 

57 

44 

45 

96 

92 

89 

85 

82 

78 

75 

71 

67 

64 

61 

58 

45 

46 

96 

93 

89 

85 

82 

79 

75 

72 

68 

65 

61 

58 

46 

47 

96 

93 

89 

86 

83 

79 

76 

72 

69 

66 

62 

59 

47 

48 

96 

93 

89 

86 

83 

79 

76 

73 

69 

66 

63 

60 

48 

49 

97 

93 

90 

86 

83 

80 

76 

73 

70 

67 

63 

60 

49 

50 

97 

93 

90 

87 

83 

80 

77 

74 

70 

67 

64 

61 

50 

51 

97 

93 

90 

87 

84 

81 

77 

74 

71 

68 

65 

62 

51 

52 

97 

94 

90 

87 

84 

81 

78 

75 

72 

69 

66 

63 

52 

53 

97 

94 

91 

87 

84 

81 

78 

75 

72 

69 

66 

65 

53 

54 

97 

94 

91 

88 

85 

82 

79 

76 

73 

70 

67 

64 

54 

55 

97 

94 

91 

88 

85 

82 

79 

76 

73 

70 

68 

65 

55 

56 

97 

94 

91 

88 

85 

82 

80 

77 

74 

71 

68 

65 

56 

57 

97 

94 

91 

88 

86 

83 

80 

77 

74 

71 

69 

66 

57 

58 

97 

94 

91 

89 

86 

83 

80 

78 

•75 

72 

69 

67 

58 

59 

97 

94 

92 

89 

86 

83 

81 

78 

75 

72 

70 

67 

59 

60 

97 

94 

92 

89 

86 

84 

81 

78 

75 

73 

70 

68 

60 

61 

97 

94 

92 

89 

87 

84 

81 

78 

76 

73 

71 

68 

61 

62 

97 

95 

92 

89 

87 

84 

81 

79 

76 

74 

71 

69 

62 

63 

97 

95 

92 

89 

87 

84 

82 

79 

77 

74 

72 

69 

63 

64 

97 

95 

9-2 

90 

87 

85 

82 

79 

77 

74 

72 

70 

64 

65 

97 

95 

92 

90 

87 

85 

82 

80 

77 

75 

72 

70 

65 

66 

97 

95 

92 

90 

87 

85 

82 

80 

78 

75 

73 

71 

66 

67 

98 

95 

93 

90 

88 

85 

83 

80 

78 

76 

73 

71 

67 

68 

98 

95 

93 

90 

88 

85 

83 

81 

78 

76 

74 

71 

68 

69 

98 

95 

93 

90 

88 

86 

83 

81 

78 

76 

74 

72 

69 

70 

98 

95 

93 

90 

88 

86 

83 

81 

79 

77 

74 

72 

70 

71 

98 

95 

93 

91 

88 

86 

84 

81 

79 

77 

75 

72 

71 

72 

98 

95 

93 

91 

88 

86 

84 

82 

79 

77 

75 

73 

72 

73 

98 

95 

93 

91 

88 

86 

84 

82 

80 

78 

75 

73 

73 

74 

98 

95 

93 

91 

88 

86 

84 

82 

80 

•78 

76 

74 

74 

75 

98 

95 

93 

91 

89 

87 

84 

82 

80 

78 

76 

74 

75 

76 

98 

95 

93 

91 

89 

87 

85 

82 

80 

78 

76 

74 

76 

77 

98 

95 

93 

91 

89 

87 

85 

83 

80 

78 

76 

74 

77 

78 

98 

96 

93 

91 

89 

87 

85 

83 

81 

79 

77 

75 

78 

79 

98 

96 

94 

91 

89 

87 

85 

83 

81 

79 

77 

75 

79 

80 

98 

96 

94 

92 

89 

87 

85 

83 

81 

79 

77 

75 

80 

t. 

0°.5 

iM 

*. 

2°.0 

2°.  5 

3°.0 

3°.  5 

4°.0 

4°.  5 

6°.0 

5°.  5 

6°.0 

.t 

9377 


84 


TABLE  II. — Relative  humidity,  per  cent. 


t 

(Dryther.) 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

t 
(Dry  ther.) 

\ 
6°.0 

6°.  5 

7°.0 

7°.5 

8°.0 

8°.5 

9°.0 

9°.5 

10°.0 

10°.  5 

11°.0 

11°.5 

12°.0 

19 

10 

19 

20 

13 

6 

20 

21 

16 

9 

2 

21 

22 

19 

12 

6 

22 

23 

21 

15 

9 

2 

23 

24 

24 

17 

11 

6 

24 

25 

26 

20 

14 

8 

3 

25 

26 

28 

23 

17 

11 

6 

26 

27 

30 

25 

19 

14 

9 

3 

27 

28 

33 

27 

22 

17 

11 

6 

1 

28 

29 

35 

29 

24 

19 

14 

9 

4 

29 

30 

36 

31 

26 

22 

17 

12 

7 

2 

30 

31 

38 

33 

29 

24 

19 

14 

10 

5 

31 

32 

40 

35 

31 

26 

21 

17 

12 

8 

3  ' 

32 

33 

42 

37 

33 

28 

24 

19 

15 

10 

6 

2 

33 

34 

44 

39 

35 

30 

26 

21 

17 

13 

9 

4 

34 

35 

45 

41 

37 

32 

28 

24 

19 

15 

12 

7 

3 

35 

36 

47 

43 

38 

34 

30 

26 

22 

18 

14 

10 

6 

2 

36 

37 

48 

44 

40 

36 

32' 

28 

24 

20 

16 

12 

8 

5 

1 

37 

38 

50 

46 

42 

38 

34 

30 

26 

22 

18 

15 

11 

7 

3 

38 

39 

52 

48 

44 

40 

36 

32 

28 

24 

20 

17 

13 

9 

6 

39 

40 

53 

49 

45 

41 

38 

34 

30 

26 

22 

19 

16 

12 

8 

40 

41 

54 

50 

46 

43 

39 

36 

32 

29 

24 

21 

18 

14 

10 

41 

42 

55 

51 

48 

44 

40 

37 

34 

30 

27 

23 

20 

16 

13 

42 

43 

56 

52 

49 

46 

42 

38 

35 

32 

29 

25 

22 

19 

15 

43 

44 

57 

53 

50 

47 

43 

40 

37 

33 

30 

27 

24 

21 

17 

44 

45 

58 

54 

51 

48 

44 

41 

38 

35 

32 

29 

25 

22 

19 

45 

46 

58 

55 

52 

49 

46 

42 

39 

36 

33 

30 

27 

23 

21 

46 

47 

59 

56 

63 

50 

47 

44 

40 

38 

34 

31 

28 

25 

22 

47 

48 

60 

56 

53 

51 

48 

45 

42 

39 

36 

33 

30 

27 

24 

48 

49 

60 

57 

54 

52 

49 

46 

43 

40 

37 

34 

31 

29 

26 

49 

50 

61 

58 

55 

52 

50 

47 

44 

41 

38 

36 

33 

30 

27 

50 

51 

62 

69 

56 

53 

50 

48 

45 

42 

39 

37 

34 

31 

28 

51 

52 

63 

60 

57 

54 

51 

48 

46 

43 

40 

38 

35 

33 

30 

52 

53 

63 

61 

58 

55 

52 

49 

47 

44 

42 

39 

36 

34 

31 

53 

54 

64 

61 

59 

56 

53 

50 

48 

45 

43 

40 

38 

35 

32 

54 

55 

65 

62 

59 

57 

54 

51 

49 

46 

43 

41 

39 

36 

34 

55 

56 

65 

63 

60 

57 

55 

52 

50 

47 

44 

42 

40 

37 

35 

56 

57 

66 

64 

61 

58 

55 

53 

50 

48 

45 

43 

40 

38 

36 

57 

58 

67 

64 

61 

59 

56 

53 

51 

49 

46 

44 

42 

39 

37 

58 

69 

67 

65 

62 

60 

57 

54 

52 

49 

47 

45 

43 

40 

38 

59 

60 

68 

65 

63 

60 

58 

55 

53 

50 

48 

46 

44 

41 

39 

60 

61 

68 

66 

63 

61 

58 

56 

54 

51 

49 

47 

44 

42 

40 

61 

62 

69 

66 

64 

61 

59 

57 

54 

52 

50 

47 

45 

43 

41 

62 

63 

69 

67 

64 

62 

60 

57 

55 

53 

51 

48 

46 

44 

42 

63 

64 

70 

67 

65 

62 

60 

58 

56 

53 

51 

49 

47 

45 

43 

64 

65 

70 

68 

65 

63 

61 

59 

56 

54 

52 

50 

48 

46 

44 

65 

66 

71 

68 

66 

63 

61 

59 

57 

55 

53 

51 

49 

47 

45 

66 

67 

71 

69 

66 

64 

62 

60 

58 

55 

53 

51 

49 

47 

45 

67 

68 

71 

69 

67 

65 

63 

60 

58 

56 

54 

52 

50 

48 

46 

68 

69 

72 

70 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

47 

69 

70 

72 

70 

68 

66 

64 

62 

60 

57 

55 

53 

52 

50 

48 

70 

71 

72 

70 

68 

66 

64 

62 

60 

58 

56 

54 

52 

50 

48 

71 

72 

73 

71 

69 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

72 

73 

73 

71 

69 

67 

65 

63 

61 

59 

57 

55 

53 

52 

50 

73 

74 

74 

72 

70 

68 

66 

64 

62 

60 

58 

56 

54 

52 

50 

74 

75 

74 

72 

70 

68 

66 

64 

62 

60 

58 

56 

55 

53 

51 

75 

76 

74 

72 

70 

68 

66 

64 

63 

61 

sa 

57 

55 

53 

52 

76 

77 

74 

73. 

71 

69 

67 

65 

63 

61 

59 

57 

56 

54 

52 

77 

78 

75 

73 

71 

69 

67 

65 

63 

62 

60 

58 

56 

54 

53 

78 

79 

75 

73 

71 

70 

68 

66 

64 

62 

60 

58 

57 

55 

53 

79 

80 

75 

73 

72 

70 

68 

66 

64 

63 

61 

59 

57 

55 

54 

80 

f. 

6°.0 

6°.5 

7e.O 

7°.5 

8°.0 

8°.5 

9°.0 

9°.5 

10°.0 

10°.5 

11°.0 

11°.5 

12°.0 

t. 

85 


TABLE  II. — Relative  humidity,  per  cent. 


I 

Difference  between  the  dry  and  wet  thermometers  (<—  O. 

|j 

0 

JS 

(^ 

12°.  0 

12°.  5 

13°.  0 

13°.  5 

14°.  0 

14°.  5 

15°.  0 

15°.  5 

16°.  0 

16°.  5 

17°.  0 

17°.  5 

18°.  0 

>> 

«. 

40 

8 

5 

1 

40 

41 

10 

7 

4 

41 

42 

13 

10 

6 

3 

42 

43 

15 

12 

9 

5 

2 

43 

44 

17 

14 

11 

8 

5 

1 

44 

45 

19 

16 

13 

10 

7 

4 

1 

45 

46 

21 

18 

15 

12 

9 

6 

3 

46 

47 

22 

20 

16 

14 

11 

8 

5 

3 

47 

48 

24 

21 

19 

16 

13 

10 

7 

5 

2 

48 

49 

26 

23 

20 

17 

15 

12 

9 

7 

4 

1 

49 

50 

27 

24 

22 

19 

16 

14 

11 

9 

6 

4 

1 

50 

61 

28 

26 

23 

21 

18 

16 

13 

10 

8 

5 

3 

0 

51 

52 

30 

27 

24 

22 

20 

17 

15 

12 

10 

7 

5 

2 

52 

63 

31 

29 

26 

24 

21 

19 

16 

14 

12 

9 

7 

4 

2 

53 

54 

32 

30 

28 

25 

23 

20 

18 

15 

13 

11 

8 

6 

4 

54 

55 

34 

31 

29 

26 

24 

22 

19 

17 

15 

12 

10 

8 

6 

55 

56 

35 

33 

30 

28 

25 

23 

21 

19 

16 

14 

12 

10 

8 

56 

57 

36 

34 

32 

29 

27 

24 

22 

20 

18 

16 

13 

11 

9 

57 

58 

37 

35 

33 

30 

28 

26 

24 

21 

19 

17 

15 

13 

11 

58 

59 

38 

36 

34 

31 

29 

27 

25 

23 

21 

18 

16 

14 

12 

59 

60 

39 

37 

34 

32 

30 

28 

26 

24 

22 

20 

18 

16 

14 

60 

61 

40 

38 

35 

33 

32 

29 

27 

25 

23 

21 

19 

17 

15 

61 

62 

41 

39 

37 

34 

32 

30 

28 

26 

24 

22 

20 

18 

16 

62 

63 

42 

40 

38 

36 

33 

31 

29 

28 

26 

24 

22 

20 

18 

63 

64 

43 

41 

38 

36 

34 

32 

30 

29 

27 

25 

23 

21 

19 

64 

65 

44 

42 

39 

37 

35 

33 

31 

29 

28 

26 

24 

22 

20 

65 

66 

45 

42 

40 

38 

36 

34 

32 

30 

29 

27 

25 

23 

22 

66 

67 

45 

43 

41 

39 

37 

35 

33 

32 

30 

28 

26 

25 

23 

67 

68 

46 

44 

42 

40 

38 

36 

34 

33 

31 

29 

27 

26 

24 

68 

69 

47 

45 

43 

41 

39 

37 

35 

33 

32 

30 

28 

26 

25 

69 

70 

48 

46 

44 

42 

40 

38 

36 

34 

33 

31 

29 

27 

26 

70 

71 

48 

46 

45 

43 

41 

39 

37 

.35 

34 

32 

30 

28 

27 

71 

72 

49 

47 

45 

43 

42 

40 

38 

36 

35 

33 

31 

30 

28 

72 

73 

50 

48 

46 

44 

42 

41 

39 

37 

35 

34 

32 

30 

29 

73 

74 

50 

48 

47 

45 

43 

41 

40 

38 

36 

35 

33 

31 

30 

74 

75 

51 

49 

47 

46 

44 

42 

40 

39 

37 

35 

34 

32 

31 

75 

76 

52 

50 

48 

46 

45 

43 

41 

39 

38 

36 

35 

33 

31 

76 

77 

52 

50 

49 

47 

45 

44 

42 

40 

39 

37 

35 

34 

32 

77 

78 

53 

51 

49 

48 

46 

44 

43 

41 

39 

38 

36 

35 

33 

78 

79 

53 

52 

50 

48 

47 

45 

43 

42 

40 

39 

37 

36 

34 

79 

80 

54 

52 

51 

49 

47 

45 

44 

42 

41 

39 

38 

36 

35 

80 

fc 

12°.  0 

12°.  5 

13°.  0 

13°.  5 

14°.  0 

14°.  5 

15°.  0 

15°.  5 

16°.  0 

16°.  5 

17°.  0 

17°.  5 

18°.  0 

t. 

TABLE  II. — Relative  humidity,  per  cent. 


? 

Difference  between  the  dry  and  wet  thermometers  (1  —  <'). 

y 

2 

1 

18°.0 

19°.0 

20°.0 

21°.0 

22°.  0 

23°.0 

24°.0 

25°.0 

26°.0 

27°.0 

28°.0 

29°.(> 

30°.0 

*j& 

«, 

55 

6 

1 

55 

50 

8 

3 

56 

57 

9 

5 

57 

58 

11 

7 

2 

58 

59 

12 

8 

4 

59 

60 

14 

10 

6 

2 

60 

61 

15 

11 

7 

3 

61 

62 

16 

13 

9 

5 

1 

62 

63 

18 

14 

10 

7 

3 

63 

64 

19 

15 

12 

8 

5 

1 

64 

65 

20 

17 

13 

10 

6 

3 

65 

66 

22 

18 

14 

11 

8 

4 

1 

66 

67 

23 

19 

16 

12 

9 

6 

2 

67 

68 

24 

20 

17 

14 

10 

7 

4 

1 

68 

69 

25 

22 

18 

15 

12 

8 

5 

2 

69 

70 

26 

23 

19 

16 

13 

10 

7 

4 

1 

70 

71 

27 

24 

20 

17 

14 

11 

8 

5 

2 

71 

72 

28 

24 

22 

18 

15 

12 

9 

6 

3 

1 

72 

73 

29 

25 

22 

19 

16 

13 

10 

8 

5 

2 

73 

74 

30 

26 

23 

20 

18 

15 

12 

9 

6 

3 

1 

74 

75 

31 

27 

24 

21 

19 

16 

13 

10 

7 

5 

2 

75 

76 

31 

28 

25 

22 

20 

17 

14 

11 

8 

6 

3 

1 

76 

77 

32 

29 

26 

23 

20 

18 

15 

12 

10 

7 

4 

2 

77 

78 

33 

30 

27 

24 

21 

19 

16 

13 

11 

8 

6 

3 

1 

78 

79 

34 

31 

28 

25 

22 

19 

17 

14 

12 

9 

7 

4 

2 

79 

80 

35 

32 

29 

26 

23 

20 

18 

15 

13 

10 

8 

6 

3 

80 

t. 

18°.0 

19°.0 

20°.0 

21°.0 

22°.  0 

23°.  0 

24°.  0 

25°.  0 

26°.  0 

27°.0 

28°.0 

29°.0 

303.0 

t. 

87 


TABLE  II. — Relative  humidity,  per  cent. 


0) 

Difference  between  the  dry  and  wet  thermometers  (t  —  t'). 

1 

ft 

10.0 

2J.0 

3°.0 

4°.0 

5°.0 

6°.0 

7°.0 

8°.0 

9°.0 

10°.0 

mo 

12°.0 

"1 

80 

96 

92 

87 

83 

79 

75 

72 

68 

64 

61 

57 

54 

80 

81 

96 

92 

88 

84 

80 

76 

72 

68 

65 

61 

58 

54 

81 

82 

96 

92 

88 

84 

80 

76 

72 

69 

65 

62 

58 

55 

82 

83 

96 

92 

88 

84 

80 

76 

73 

69 

66 

62 

59 

55 

83 

84 

96 

92 

88 

84 

80 

77 

73 

69 

66 

63 

59 

56 

84 

85 

96 

92 

88 

84 

80 

77 

73 

70 

66 

63 

60 

56 

85 

86 

96 

92 

88 

84 

81 

77 

73 

70 

67 

63 

60 

57 

86 

87 

96 

92 

88 

84 

81 

77 

74 

70 

67 

64 

60 

57 

87 

88 

96 

92 

88 

85 

81 

77 

74 

71 

67 

64 

61 

58 

88 

89 

96 

92 

88 

85 

81 

78 

74 

71 

68 

64 

61 

58 

89 

90 

96 

92 

88 

85 

81 

78 

75 

71 

68 

65 

62 

59 

90 

91 

96 

92 

89 

85 

82 

78 

75 

71 

68 

65 

62 

59 

91 

92 

96 

92 

89 

85 

82 

78 

75 

72 

69 

65 

62 

59 

92 

93 

96 

93 

89 

85 

82 

78 

75 

72 

69 

66 

63 

60 

93 

94 

96 

93 

89 

86 

82 

79 

75 

72 

69 

66 

63 

60 

94 

95 

96 

93 

89 

86 

82 

79 

76 

72 

69 

66 

63 

60 

95 

96 

96 

93 

89 

86 

82 

79 

76 

73 

70 

67 

64 

61 

96 

97 

96 

93 

89 

86 

82 

79 

76 

73 

70 

67 

64 

61 

97 

98 

96 

93 

89 

86 

83 

79 

76 

73 

70 

67 

64 

61 

98 

99 

96 

93 

89 

86 

83 

80 

76 

73 

70 

68 

65 

62 

99 

100 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

65 

62 

100 

101 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

65 

62 

101 

102 

97 

93 

90 

86 

83 

80 

77 

74 

71 

68 

65 

63 

102 

103 

97 

93 

90 

87 

83 

80 

77 

74 

71 

69 

66 

63 

103 

104 

97 

93 

90 

87 

83 

80 

77 

74 

72 

69 

66 

63 

104 

105 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

66 

64 

105 

106 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

66 

64 

106 

107 

97 

93 

90 

87 

84 

81 

78 

75 

72 

69 

67 

64 

107 

108 

97 

93 

90 

87 

84 

81 

78 

75 

72 

70 

67 

64 

108 

109 

97 

93 

90 

87 

84 

81 

78 

75 

73 

70 

67 

65 

109 

110 

97 

94 

90 

87 

84 

81 

78 

76 

73 

70 

67 

65 

110 

111 

97 

94 

90 

87 

84 

81 

78 

76 

73 

70 

68 

65 

111 

112 

97 

94 

90 

87 

84 

82 

79 

76 

73 

71 

68 

65 

112 

113 

97 

94 

90 

87 

85 

82 

79 

76 

73 

71 

68 

66 

113 

114 

97 

94 

91 

88 

85 

82 

79 

76 

74 

71 

68 

66 

114 

115 

97 

94 

91 

88 

85 

82 

79 

76 

74 

71 

69 

66 

115 

116 

97 

94 

91 

88 

85 

82 

79 

76 

74 

71 

69 

66 

116 

117 

97 

94 

91 

88 

85 

82 

79 

77 

74 

71 

69 

66 

117 

118 

97 

94 

91 

88 

85 

82 

79 

77 

74 

72 

69 

67 

118 

119 

97 

94 

91 

88 

85 

82 

80 

77 

74 

72 

69 

67 

119 

120 

97 

94 

91 

88 

85 

83 

80 

77 

75 

72 

70 

67 

120 

121 

97 

94 

91 

88 

85 

83 

80 

77 

75 

72 

70 

67 

121 

122 

97 

94 

91 

88 

85 

83 

80 

77 

75 

72 

70 

67 

122 

123 

97 

94 

91 

88 

85 

83 

80 

78 

75 

73 

70 

68 

123 

124 

97 

94 

91 

88 

85 

83 

80 

78 

75 

73 

70 

68 

124 

125 

97 

94 

91 

88 

86 

83 

80 

78 

75 

73 

70 

68 

125 

126 

97 

94 

91 

88 

86 

83 

80 

78 

75 

73 

71 

68 

126 

127 

97 

94 

91 

89 

86 

83 

81 

78 

76 

73 

71 

68 

127 

128 

97 

94 

91 

89 

86 

83 

81 

78 

76 

73 

71 

68 

128 

1*9 

97 

94 

91 

89 

86 

83 

81 

78 

76 

73 

71 

69 

129 

130 

97 

94 

91 

89 

86 

83 

81 

78 

76 

74 

71 

69 

130 

131 

97 

94 

92 

89 

86 

84 

81 

79 

76 

74 

71 

69 

131 

132 

97 

94 

92 

89 

86 

84 

81 

79 

76 

74 

72 

69 

132 

133 

97 

94 

92 

89 

86 

84 

81 

79 

76 

'  74 

72 

69 

133 

134 

97 

94 

92 

89 

86 

84 

81 

79 

76 

74 

72 

70 

134 

135 

97 

94 

92 

89 

86 

84 

81 

79 

77 

74 

72 

70 

135 

136 

97 

94 

92 

89 

87 

84 

82 

79 

77 

74 

72 

70 

136 

137 

97 

94 

92 

89 

87 

84 

82 

79 

77 

74 

72 

70 

137 

138 

97 

95 

92 

89 

87 

84 

82 

79 

77 

74 

73 

70 

138 

139 

97 

95 

92 

89 

87 

84 

82 

79 

77 

75 

73 

70 

139 

140 

97 

95 

92 

89 

87 

84 

82 

79 

77 

75 

73 

71 

140 

t. 

1... 

2°.0 

3°.0 

4°.0 

5°.0 

6°.0 

7°.0 

8°.0 

9°.0 

10°.0 

mo 

12°.0 

t. 

88 


TABLE  II. — Eelative  humidity,  per  cent. 


? 

Difference  between  the  dry  and  wet  thermometers  (  t  —  tf). 

? 

£ 

J9 

•»-  +- 

~I 

12°.0 

13°.0 

14°.0 

15°.0 

16°.0 

17°.0 

18°.0 

19°.0 

20°.  0 

21°.0 

22°.0 

23°.0 

24°.0 

| 

80 

54 

51 

47 

44 

41 

38 

35 

32 

29 

26 

23 

20 

18 

80 

81 

54 

51 

48 

44 

41 

38 

35 

33 

30 

27 

24 

21 

19 

81 

82 

55 

52 

48 

45 

42 

39 

36 

33 

31 

28 

25 

22 

20 

82 

83 

55 

52 

49 

46 

43 

40 

37 

34 

31 

29 

26 

23 

21 

83 

84 

56 

53 

49 

4G 

44 

41 

38 

35 

32 

29 

27 

24 

22 

84 

85 

56 

53 

50 

47 

44 

41 

38 

36 

33 

30 

28 

25 

22 

85 

86 

57 

54 

51- 

48 

45 

42 

39 

36 

34 

31 

29 

26 

23 

86 

87 

57 

54 

51 

48 

45 

42 

40 

37 

34 

32 

30 

27 

24 

87 

88 

58 

55 

52 

49 

46 

43 

40 

38 

35 

32 

30 

27 

25 

88 

89 

58 

55 

52 

49 

46 

44 

41 

38 

36 

33 

31 

28 

26 

89 

90 

59 

56 

53 

50 

47 

44 

41 

39 

36 

34 

32 

29 

26 

90 

91 

59 

56 

53 

50 

47 

45 

42 

39 

37 

35 

33 

30 

27 

91 

92 

59 

56 

54 

51 

48 

45 

43 

40 

37 

35 

33 

30 

28 

92 

93 

60 

57 

54 

51 

48 

46 

43 

41 

38 

36 

34 

31 

29 

93 

94 

60 

57 

54 

52 

49 

46 

44 

41 

39 

36 

34 

31 

29 

94 

95 

60 

58 

55 

52 

49 

47 

44 

42 

39 

37 

35 

32 

30 

95 

96 

61 

58 

55 

53 

50 

47 

45 

42 

40 

37 

36 

33 

30 

96 

97 

61 

58 

56 

53 

50 

48 

45 

43 

40 

38 

36 

33 

31 

97 

98 

61 

59 

56 

53 

51 

48 

46 

43 

41 

38 

37 

34 

32 

*98 

99 

62 

59 

56 

54 

51 

49 

46 

44 

41 

39 

37 

34 

32 

99 

100 

62 

59 

57 

54 

51 

49 

47 

44 

42 

39 

37 

35 

33 

100 

101 

62 

60 

57 

54 

52 

49 

47 

45 

42 

40 

38 

36 

33 

101 

102 

63 

60 

57 

55 

52 

50 

47 

45 

43 

40 

38 

36 

34 

102 

103 

63 

60 

58 

55 

53 

50 

48 

45 

43 

41 

39 

37 

34 

103 

104 

63 

61 

58 

55 

53 

51 

48 

46 

44 

41 

39 

37 

35 

104 

105 

64 

61 

58 

56 

53 

51 

49 

46 

44 

42 

40 

38 

35 

105 

106 

64 

61 

59 

56 

54 

51 

49 

47 

44 

42 

40 

38 

36 

106 

107 

64 

62 

59 

57 

54 

52 

49 

47 

45 

43 

41 

38 

36 

107 

108 

64 

62 

59 

57 

64 

52 

50 

47 

45 

43 

41 

39 

37 

108 

109 

65 

62 

60 

57 

55 

52 

50 

48 

46 

44 

41 

39 

37 

109 

110 

65 

62 

60 

57 

55 

53 

50 

48 

46 

44 

42 

40 

38 

110 

111 

65 

63 

60 

58 

55 

53 

51 

49 

46 

44 

42 

40 

38 

111 

112 

65 

63 

60 

58 

56 

53 

51 

49 

47 

45 

43 

40 

39 

112 

113 

66 

63 

61 

58 

56 

54 

51 

49 

47 

45 

43 

41 

39 

113 

114 

66 

63 

61 

59 

56 

54 

52 

50 

48 

45 

44 

41 

39 

114 

115 

66 

64 

61 

59 

57 

54 

52 

50 

48 

46 

44 

42 

40 

115 

116 

66 

64 

61 

59 

57 

55 

52 

50 

48 

46 

44 

42 

40 

116 

117 

66 

64 

62 

59 

57 

55 

53 

51 

49 

46 

44 

43 

41 

117 

118 

67 

.64 

62 

60 

57 

55 

53 

51 

49 

47 

45 

43 

41 

118 

119 

67 

64 

62 

60 

58 

55 

53 

51 

49 

47 

45 

43 

41 

119 

120 

67 

65 

62 

60 

58 

56 

54 

51 

49 

47 

45 

44 

42 

120 

121 

67 

65 

63 

60 

58 

56 

54 

52 

50 

48 

46 

44 

42 

121 

122 

67 

65 

63 

61 

58 

56 

54 

52 

50 

48 

46 

44 

42 

122 

123 

68 

65 

63 

61 

59 

57 

54 

52 

50 

48 

46 

45 

43 

123 

124 

68 

66 

63 

61 

59 

57 

55 

53 

51 

49 

47 

45 

43 

124 

125 

68 

66 

64 

62 

59 

57 

55 

53 

51 

49 

47 

45 

43 

125 

126 

68 

66 

64 

62 

59 

57 

55 

53 

51 

49 

47 

46 

44 

126 

127 

68 

66 

64 

62 

60 

58 

55 

54 

52 

50 

48 

46 

44 

127 

128 

68 

66 

64 

62 

60 

58 

56 

54 

52 

50 

48 

46 

44 

128 

129 

69 

66 

64 

62 

60 

58 

56 

54 

52 

50 

48 

46 

45 

129 

130 

69 

67 

65 

62 

60 

58 

56 

54 

52 

50 

49 

47 

45 

130 

131 

69 

67 

65 

63 

60 

58 

56 

54 

53 

51 

49. 

47 

45 

131 

132 

69 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

47 

46 

132 

133 

69 

67 

65 

63 

61 

59 

57 

55 

53 

51 

49 

48 

46 

133 

134 

70 

67 

65 

63 

61 

59 

57 

55 

53 

51 

50 

48 

46 

134 

135 

70 

68 

65 

63 

61 

59 

57 

55 

53 

51 

50 

48 

46 

135 

136 

70 

68 

66 

64 

61 

59 

58 

56 

54 

52 

50 

48 

47 

136 

137 

70 

68 

66 

64 

62 

60 

58 

56 

54 

52 

50 

49 

47 

137 

138 

70 

68 

66 

64 

62 

60 

58 

56 

54 

52 

51 

49 

47 

138 

139 

70 

68 

66 

64 

62 

60 

58 

56 

54 

53 

51 

49 

47 

139 

140 

71 

68 

66 

64 

62 

60 

58 

56 

55 

53 

51 

49 

48 

140 

t. 

12°.0 

13°.0 

14°.0 

15°.0 

16°.0 

17°.0 

18°.0 

19°.0 

20°.0 

21°.0 

22°.0 

23°.0 

24°.0 

t. 

89 


TABLE  II. — Relative  humidity,  per  cent. 


.1 

Difference  between  the  dry  and  wet  thermometers  (t  —  if). 

I 

4°.0 

25°.0 

26°.0 

27°.0 

8°.0 

29°.0 

30°.0 

31°.0 

32°.0 

33°.0 

34°.0 

35°.0 

36°.0 

~| 

80 

18 

15 

13 

10 

8 

6 

3 

1 

80 

81 

19 

16 

14 

11 

9 

7 

4 

2 

81 

82 

20 

17 

15 

12 

10 

8 

5 

3 

1 

82 

83 

21 

18 

16 

13 

11 

9 

6 

4 

2 

83 

84 

22 

19 

17 

14 

12 

10 

8 

5 

3 

1 

84 

85 

22 

20 

17 

15 

13 

11 

9 

6 

4 

2 

85 

86 

23 

21 

18 

16 

14 

12 

10 

7 

5 

3 

•1 

86 

87 

24 

22 

19 

17 

15 

13 

11 

8 

6 

4 

2 

87 

88 

25 

22 

20 

18 

16 

14 

12 

9 

7 

5 

3 

1 

88 

89 

26 

23 

21 

19 

16 

14 

12 

10 

8 

6 

4 

2 

1 

89 

90 

26 

24 

22 

20 

17 

15 

13 

11 

9 

7 

5 

3 

2 

90 

91 

27 

25 

23 

20 

18 

16 

14 

12 

10 

8- 

6 

4 

3 

91 

92 

28 

26 

23 

21 

19 

17 

15 

13 

11 

9 

7 

5 

3 

92 

93 

29 

26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

93 

94 

29 

27 

25 

23 

21 

18 

16 

14 

13 

11 

9 

7 

5 

94 

95 

30 

28 

25 

23 

21 

19 

17 

15 

13 

11 

10 

8 

6 

95 

96 

30 

28 

26 

24 

22 

20 

18 

16 

14 

12 

10 

9 

7 

96 

97 

31 

29 

27 

25 

23 

21 

19 

17 

15 

13 

11 

10 

8 

97 

98 

32 

29 

27 

25 

23 

21 

19 

18 

16 

14 

12 

10 

9 

98 

99 

32 

30 

28 

26 

24 

22 

20 

18 

16 

15 

13 

11 

10 

99 

100 

33 

31 

29 

27 

25 

23 

21 

19 

17 

15 

14 

12 

10 

100 

101 

33 

31 

29 

27 

25 

23 

21 

20 

18 

16 

14 

13 

11 

101 

102 

34 

32 

30 

28 

26 

24 

22 

20 

19 

17 

15 

13 

12 

102 

103 

34 

32 

30 

28 

26 

25 

23 

21 

19 

17 

16 

14 

12 

103 

104 

35 

33 

31 

29 

27 

25 

23 

22 

20 

18 

16 

15 

13 

104 

105 

35 

33 

31 

30 

28 

26 

24 

22 

20 

19 

17 

15 

14 

105 

106 

36 

34 

32 

30 

28 

26 

25 

23 

21 

19 

18 

16 

14 

106 

107 

36 

34 

32 

31 

29 

27 

25 

23 

22 

20 

18 

17 

15 

107 

108 

37 

35 

33 

31 

29 

27 

26 

24 

22 

21 

19 

17 

16 

108 

109 

37 

35 

33 

32 

30 

28 

26 

25 

23 

21 

20 

18 

16 

109 

110 

38 

36 

34 

32 

30 

28 

27 

25 

23 

22 

20 

19 

17 

110 

111 

38 

36 

34 

33 

31 

29 

27 

26 

24 

22 

21 

19 

18 

111 

112 

39 

37 

35 

33 

31 

29 

28 

26 

24 

23 

21 

20 

18 

112 

113 

39 

37 

35 

33 

32 

30 

28 

27 

25 

23 

22 

20 

19 

113 

114 

39 

38 

36 

34 

32 

30 

29 

27 

25 

24 

22 

21 

19 

114 

115 

40 

38 

36 

34 

33 

31 

29 

28 

26 

24 

23 

21 

20 

115 

116 

40 

39 

37 

35 

33 

31 

30 

28 

26 

25 

23 

22 

20 

116 

117 

41 

39 

37 

35 

33 

32 

30 

29 

27 

25 

24 

22 

21 

117 

118 

41 

39 

37 

36 

34 

32 

31 

29 

27 

26    24 

23  . 

21 

118 

119 

41 

40 

38 

36 

34 

33 

31 

29 

28 

26 

25 

23 

22 

119 

120 

42 

40 

38 

36 

35 

33 

31 

30 

28 

27 

25 

24 

22 

120 

121 

42 

40 

38 

37 

35 

33 

32 

30 

29 

27 

26 

24 

23 

121 

122 

42 

41 

39 

37 

35 

34 

32 

31 

29 

28 

26 

25 

23 

122 

123 

43 

41 

39 

37 

36 

34 

33 

31 

29 

28 

26 

25 

24 

123 

124 

43 

41 

40 

38 

36 

35 

33 

31 

30 

28 

27 

25 

24 

124 

125 

43 

42 

40 

38 

37 

35 

33 

32 

30 

29 

27 

26 

24 

125 

126 

44 

42- 

40 

39 

37 

35 

34 

32 

31 

29 

28 

.  26 

25 

126 

127 

44 

42 

41 

39 

37 

36 

34 

32 

31 

30 

28 

27 

25 

127 

128 

44 

43 

41 

39 

38 

36 

34 

33 

31 

30 

29 

27 

26 

128 

129 

45 

43 

41 

40 

38 

36 

35 

33 

32 

30 

29 

28 

26 

129 

130 

45 

43 

42 

40 

38 

37 

35 

33 

32 

31 

29 

28 

27 

130 

131 

45 

44 

42 

40 

39 

37 

35 

34 

32 

31 

30 

28 

27 

131 

132 

46 

44 

42 

40 

39 

37 

36 

34 

33 

31 

30 

29 

27 

132 

13' 

46 

44 

42 

41 

39 

38 

36 

35 

33 

32 

30 

29 

28 

133 

134 

46 

44 

43 

41 

39 

38 

36 

35 

33 

32 

31 

29 

28 

134 

135 

46 

45 

43 

41 

40 

38 

37 

35 

34 

32 

31 

30 

28 

135 

136 

47 

45 

43 

42 

40 

39 

37 

36 

34 

33 

31 

30 

29 

136 

13' 

47 

45 

44 

42 

40 

39 

37 

36 

34 

33 

32 

30 

29 

137 

138 
139 

47 
47 

45 
46 

44 
44 

42 
42 

41 
41 

39 

39 

38 
38 

36 

37 

35 
35 

33 
34 

32 
32 

31 
31 

29 
30 

138 
139 

140 

48 

46 

44 

43 

41 

40 

38 

37 

35 

34 

33 

31 

30 

140 

t. 

24°.0 

25°.0 

26°.0 

27c.O 

28°.0 

29°.  0 

30°.  0 

31°.  0 

32°.0 

33°.0 

34°.  0 

35°.0 

36°.  0 

t. 

90 


TABLE  II. — Relative  humidity,  per  cent. 


J 

Difference  between  the  dry  and  wet  thermometers  (t  —  f). 

J 

b 

e^ 

36°.  0 

37°.  0 

38°.  0 

39°.  0 

40°.  0 

41°.  0 

42°.  0 

43°.  0 

44°.  0 

45°.  0 

46°.  0 

47°.  0 

48°.  0 

b 

0, 

89 

1 

89 

90 

2 

90 

91 

3 

1 

91 

92 

3 

2 

1 

92 

93 

4 

3 

2 

93 

94 

5 

4 

8 

0 

94 

95 

6 

5 

4 

1 

95 

96 

7 

5 

5 

2 

1 

96 

97 

8 

6 

6 

3 

1 

97 

98 

9 

7 

7 

4 

2 

1 

98 

99 

10 

8 

7 

5 

3 

2 

0 

99 

100 

10 

9  ' 

8 

6 

4 

3 

1 

100 

301 

11 

9 

9 

6 

5 

3 

2 

0 

101 

102 

12 

10 

9 

7 

6 

4 

3 

1 

102 

103 

12 

11 

9 

8 

6 

5 

4 

2 

1 

103 

104 

13 

12 

10 

8 

7 

6 

5 

3 

2 

104 

105 

14 

12 

11 

9 

8 

6 

6 

4 

2 

1 

105 

106 

14 

13 

11 

10 

8 

7 

6 

4 

3 

2 

0 

106 

107 

15 

14 

12 

11 

9 

8 

6 

5 

4 

3 

1 

107 

108 

16 

14 

13 

11 

10 

9 

7 

6 

5 

3 

2 

1 

108 

109 

16 

15 

13 

12 

10 

9 

8 

7 

5 

4 

3 

1 

109 

110 

17 

15 

14 

13 

11 

10 

8 

7 

6 

5 

3 

2 

1 

110 

111 

18 

16 

15 

13 

12 

10 

9 

8 

7 

5 

4 

3 

2 

111 

112 

18 

17 

15 

14 

12 

11 

10 

8 

7 

6 

5 

4 

2 

112 

113 

19 

17 

16 

14 

13 

12 

10 

9 

8 

7 

5 

4 

3 

113 

114 

19 

18 

16 

15 

14 

12 

11 

10 

8 

7 

6 

5 

4 

114 

115 

20 

18 

17 

16 

14 

13 

12 

10 

9 

8 

7 

6 

4 

115 

116 

20 

19 

17 

16 

15 

13 

12 

11 

10 

8 

7 

6 

5 

116 

117 

21 

19 

18 

17 

15 

14 

13 

11 

10 

9 

8 

7 

6 

117 

118 

21 

20 

18 

17 

16 

15 

13 

12 

11 

10 

8 

7 

6 

118 

119 

22 

20 

19 

18 

16 

15 

14 

12 

11 

10 

9 

8 

7 

119 

120 

22 

21 

19 

18 

17 

16 

14 

13 

12 

11 

10 

9 

8 

120 

121 

23 

21 

20 

18 

17 

16 

15 

13 

12 

11 

10 

9 

8 

121 

122 

23 

22 

20 

19 

18 

17 

15 

14 

13 

12 

11 

10 

9 

122 

123 

24 

22 

21 

19 

18 

17 

16 

14 

13 

12 

11 

10 

9 

123 

124 

24 

23 

21 

20 

19 

18 

16 

15 

14 

13 

12 

11 

9 

124 

125 

24 

23 

22 

20 

19 

18 

17 

15 

14 

13 

12 

11 

10 

125 

126 

25 

23 

22 

21 

20 

19 

17 

16 

15 

14 

13 

12 

10 

126 

127 

25 

24 

23 

21 

20 

19 

18 

16 

15 

14 

13 

12 

11 

127 

128 

26 

24 

23 

22 

21 

19 

18 

17 

16 

15 

14 

13 

11 

128 

129 

26 

25 

24 

22 

21 

20 

19 

17 

16 

15 

14 

13 

12 

129 

130 

27 

25 

24 

23 

21 

20 

19 

18 

17 

16 

15 

14 

12 

130 

131 

27 

26 

24 

23 

22 

21 

19 

18 

17 

16 

15 

14 

13 

131 

132 

27 

26 

25 

23 

22 

21 

20 

19 

18 

16 

15 

14 

13 

132 

133 

28 

26 

25 

24 

23 

21 

20 

19 

18 

17 

16 

15 

14 

133 

134 

28 

27 

25 

24 

23 

22 

21 

19 

18 

17 

16 

15 

14 

134 

135 

28 

27 

26 

24 

23 

22 

21 

20 

19 

18 

17 

16 

15 

135 

136 

29 

27 

26 

25 

24 

22 

21 

20 

19 

.18 

17 

16 

15 

136 

137 

29 

28 

26 

25 

24 

23 

22 

20 

19 

18 

17 

16 

15 

137 

138 

29 

28 

27 

25 

24 

23 

22 

21 

20 

19 

18 

17 

16 

138 

139 

30 

28 

27 

26 

25 

24 

22 

21 

20 

19 

18 

17 

16 

139 

140 

30 

29 

28 

2G 

25 

24 

23 

22 

21 

20 

18 

17 

16 

140 

t. 

36°.  0 

37°.  0 

38°.  0 

39°.  0 

40c.O 

41°.  0 

42°.  0 

43°.  0 

44°.  0 

45°.  0 

46°.  0 

47°.  0 

48°.  0 

t. 

91 


TABLE  III. — Correction  to  vapor  pressure  for  reading  of  barometer. 


1 

Inches. 

p 

7 

30 

29 

28 

27 

26 

25 

24 

23 

22 

21 

20 

19 

18 

T 

. 

.000 

.000 

+  .001 

+  .001 

+.001 

+  .002 

+.002 

+  .003 

+.003 

+  .003 

+  .004 

+.004 

+.004 

i 

2 

.000 

.000 

.001 

.002 

.002 

.003 

.004 

-.005 

.006 

.007 

.007 

.008 

.009 

2 

3 

—.001 

.000 

.001 

.002 

.003 

.004 

.006 

.007 

.008 

.009 

.010 

.011 

.012 

3 

4 

-.001 

.000 

.002 

.003 

.005 

.006 

.008 

.009 

.011 

.012 

.014 

.015 

.017 

4 

5 

-.001 

+  .001 

.002 

.004 

.006 

.008 

.010 

.012 

.014 

.015 

.017 

.019 

.021 

5 

G 

—  001 

001 

003 

005 

008 

010 

.012 

.014 

016 

019 

021 

.023 

025 

6 

7 

-.001 

.001 

.003 

.006 

.009 

.012 

.014 

.017 

.019 

.022 

.024 

.027 

.030 

7 

8 

-.001 

.001 

.004 

.007 

.010 

.013 

.016 

.019 

.022 

.025 

.028 

.031 

.034 

8 

9 

-.001 

.002 

.005 

.008 

.012 

.015 

.018 

.022 

.025 

.028 

.032 

.035 

.038 

9 

10 

-.002 

.002 

.005 

.009 

.013 

.017 

.020 

.024 

.027 

.031 

.035 

.039 

.043 

10 

11 

-.002 

.002 

.000 

.010 

.014 

.018 

.022 

.026 

.031 

.035 

.039 

.043 

.047 

11 

12 

-.003 

.002 

.006 

.010 

.015 

.019 

.024 

.028 

.032 

.037 

.041 

.046 

.050 

12 

13 

-.003 

'   .002 

.006 

.011 

.016 

.021 

.026 

.030 

.035 

.040 

.045 

.050 

.055 

13 

14 

-.004 

.002 

.007 

.012 

.017 

.022 

.028 

.033 

.038 

.043 

.048 

.054 

.Oo9 

14 

15 

-.004 

.002 

.007 

.013 

.019 

.024 

.030 

.035 

.041 

.046 

.052 

.058 

.063 

15 

16 

—  004 

002 

.008 

014 

020 

.026 

.032 

.038 

.044 

.049 

.055 

.061 

067 

16 

17 

-.004 

.002 

.008 

.015 

.021 

.027 

.034 

.040 

.046 

.053 

.059 

.065 

.072 

17 

18 

-.004 

.002 

.009 

.016 

.022 

.029 

.036 

.042 

.049 

.056 

.062 

.069 

.076 

18 

19 

-.005 

.002 

.009 

.017 

.024 

.031 

.038 

.045 

.052 

.059 

.066 

.073 

.080 

19 

20 

—.005 

.003 

.010 

.018 

.026 

.033 

.041 

.048 

.055 

.063 

.070 

.078 

.085 

20 

21 

—.005 

.003 

.011 

.019 

.027 

.034 

.042 

.050 

.057 

.066 

.073 

.081 

.089 

21 

22 

—.005 

.003 

.011 

.020 

.028 

.036 

.044 

.052 

.061 

.069 

.077 

.085 

.093 

22 

-23 

—.005 

.003 

.012 

.021 

.029 

.038 

.046 

.055 

.063 

.072 

.081 

.089 

.098 

23 

24 

—.005 

.004 

.013 

.021 

.030 

.039 

.048 

.057 

.066 

.075 

.084 

.093 

.102 

24 

25 

—.006 

.004 

.013 

.022 

.032 

.041 

.050 

.060 

.069 

.078 

.088 

.097 

.106 

25 

26 

—.006 

.004 

.013 

.023 

.033 

.043 

.052 

.062 

.072 

.081 

.091 

.101 

.111 

26 

27 

—.006 

.004 

.014 

.024 

.034 

.044 

.054 

.065 

.075 

.085 

.095 

.105 

.115 

27 

28 

-.006 

.004 

.015 

.025 

.036 

.046 

.056 

.067 

.077 

.088 

.098 

.109 

.119 

28 

29 

—.007 

.004 

.015 

.026 

.037 

.048 

.059 

.069 

.080 

.091 

.102 

.113 

.124 

29 

30 

—.007 

.004 

.016 

.027 

.038 

.049 

.061 

.072 

.083 

.094 

.105 

.117 

.128 

30 

31 

—.007 

.005 

.016 

.028 

.039 

.051 

.063 

.074 

.086 

.097 

.109 

.121 

.132 

31 

32 

—  .007 

.005 

.017 

.029 

.041 

.053 

.065 

.077 

.089 

.101 

.113 

.125 

.137 

32 

33 

—.007 

.005 

.017 

.030 

.042 

.054 

.067 

.079 

.092 

.104 

.116 

.129 

.141 

33 

34 

—.008 

.005 

.018 

.031 

.043 

.056 

.069 

.082 

.094 

.107 

.120 

.133 

.145 

34 

35 

—.008 

.005 

.018 

.032 

.045 

.058 

.071 

.084 

.097 

.110 

.123 

.137 

.150 

35 

36 

—.008 

.005 

.019 

.032 

.046 

.059 

.073 

.086 

.100 

.114 

.127 

.141 

.154 

36 

37 

—.008 

.006 

.019 

.033 

.047 

.061 

.075 

.089 

.103 

.117 

.131 

.145 

.158 

37 

38 

—  v.009 

.006 

.020 

.034 

.049 

.063 

.077 

.091 

.106 

.120 

.134 

.149 

.163 

38 

39 

—.009 

.006 

.021 

.035 

.050 

.065 

.079 

.094 

.109 

.123 

.138 

.153 

.167 

39 

40 

—.009 

+.006 

+.021 

+.036 

+.051 

+  .066 

+.081 

+  .096 

+.111 

+.126 

+  .142 

+.157 

+  .172 

40 

Dew  Point 
Pressure 

Correction. 

92 

TABLE  IV.— Vapor  pressure,  in  inches. 


E 

g 

i 

i 

g 

e 

g 

I 

D 

1 

1 

a 

o> 

"a 

I 

a 

o> 

c 

g 

a 

1 

"S 

E 

d 

E 

I 

h 

a 

p. 

'I 

£ 

1 

!_ 

'I 

a 

S-i 

a 

5 

a 

5 

•: 

a 

•s 

o 
0, 

y. 

1 

k 

a 

a 

o 
a 

a 

cS 

01 

0> 

cS 

cj 

o 

™ 

ft 

ps> 

ft 

^f 

ft 

£> 

ft 

n 

£ 

Q 

|^r 

ft 

—60 

.0010 

—30 

.0069 

0 

.038 

30 

.164 

60 

.517 

90 

1.408 

120 

3.425 

—59 

.0011 

—29 

.0074 

1 

.040 

31 

.172 

61 

.536 

91 

1.453 

121 

3.  522 

—58 

.0012 

—28 

.0078 

2 

.043 

32 

.180 

62 

.555 

92 

1.  499 

122 

3.621 

—57 

.0013 

—27 

.0084 

3 

.045 

33 

.187 

63 

.575 

93 

1.546 

123 

3.  723 

—56 

.0013 

—26 

.0089 

4 

.047 

34 

.195 

64 

.595 

94 

1.  595 

124 

3.827 

—55 

.0015 

—25 

.0094 

5 

.049 

35 

.203 

65 

.616 

95 

1.  645 

125 

3.933 

—54 

.0016 

—24 

.010 

6 

.052 

36 

.211 

66 

.638 

96 

1.696 

126 

4.042 

—53 

.0017 

—23 

.011 

7 

.054 

37 

.219 

67 

.660 

97 

1.749 

127 

4.154 

—52 

.0018 

—22 

.011 

8 

.057 

38 

.228 

68 

.684 

98 

1.803 

128 

4.268 

—51 

.0019 

—21 

.012 

9 

.060 

39 

.  237 

69 

.707 

99 

1.859 

129 

4.385 

—50 

.0021 

—20 

.013 

*10 

.063 

40 

.246 

70 

.732 

100 

1.916 

130 

4.504 

—49 

.0022 

—19 

.013 

11 

.067 

41 

.256 

71 

.757 

101 

1.975 

131 

4.627 

—48 

.0024 

—18 

.014 

12 

.070 

42 

.266 

72 

.783 

102 

2.035 

132 

4.  752 

—47 

.0026 

—17 

.015 

13 

.074 

43 

.277 

73 

.810 

103 

2.097 

133 

4.880 

—46 

.0027 

—16 

.016 

14 

.077 

44 

.287 

74 

.838 

104 

2.  160 

134 

5.011 

—45 

.0029 

—15 

.017 

15 

.081 

45 

.298 

75 

.866 

105 

2.225 

135 

5.145 

—44 

.00:51 

—14 

.018 

16 

.085 

46 

.310 

76 

.896 

106 

2.  292 

136 

5.282 

—43 

.0033 

—13 

.019 

17 

.089 

47 

.322 

77 

.926 

107 

2.360 

137 

5.422 

—42 

.0035 

—12 

.020 

18 

.093 

48 

.334 

78 

.957 

108 

2.431 

138 

5.565 

—41 

.0037 

—11 

.021 

19 

.098 

49 

.347 

79 

.989 

109 

2.503 

139 

5.712 

—40 

.0039 

—10 

.022 

20 

.103 

50 

.360 

80 

1.022 

110 

2.576 

140 

5.862 

—39 

.0041 

—  9 

.023 

21 

.108 

51 

.373 

81 

1.056 

111 

2.652 

—38 

.0044 

—  8 

.025 

22 

.113 

52 

.387 

82 

1.091 

112 

2.730 

—37 

.0046 

-  7 

.026 

23 

.119 

53 

.402 

83 

1.  127 

113 

2.  810 

—36 

.0048 

—  6 

.028 

24 

.125 

54 

.417 

84 

1.163 

114 

2.891 

—35 

.0051 

—  5 

.029 

25 

.130 

55 

.432 

85 

1.202 

115 

2.975 

—34 

.0054 

—  4 

.031 

26 

.137 

56 

.448 

86 

1.241 

116 

3.061 

—33 

.0058 

—  3 

.033 

27 

.143 

57 

.465 

87 

1.281 

117 

3.148 

—32 

.0061 

—  2 

.034 

28 

.150 

58 

.481 

88 

1.322 

118 

3.  239 

—31 

.0065 

—  1 

.036 

29 

.157 

59 

.499 

89 

1.364 

119 

3.331 

Vapor 

Pros- 

sure. 

93 


TABLE  V. — Grains  of  water-vapor  contained  in  a  cubic  foot  of  air. 


J 

°3 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

I 

j 

—20 

.219 

—10 

.356 

.340 

.324 

.309 

.294 

.281 

.267 

.254 

.242 

.231 

—  0 

.564 

.540 

.516 

.493 

.471 

.450 

.430 

.411 

.391 

.374 

+  o 

.564 

.590 

.617 

.645 

.674 

.705 

.735 

.767 

.801 

.837 

10 

.87C 

.910 

.950 

.991 

1.033 

1.075 

1.122 

1.169 

1.217 

1.268 

20 

1.321 

1.374 

1.430 

1.488 

1.550 

1.611 

1.675 

1.743 

1.812 

1.884 

30 

1.  958 

2.034 

2.113 

2.194 

2.279 

2.366 

2.457 

2.550 

2.646 

2.746 

40 

2.8*9 

2.955 

3.064 

3.177 

3.294 

3.414 

3.539 

3.667 

3.800 

3.936 

50 

4.076 

4.222 

4.372- 

4.526 

4.685 

4.849 

5.01& 

5.191 

5.371 

5.555 

60 

5.744 

5.941 

6.142 

6.  350 

6.563 

6.782 

7.009 

7.241 

7.480 

7.726 

70 

7.980 

8.240 

8.508 

8.782 

9.065 

9.  356 

9.655 

9.961 

10.  277 

10.601 

80 

10.  933 

11.  275 

11.626 

11.987 

12.  356 

12.  736 

13.  127 

13.  576 

13.  937 

14.358 

90 

14.  791 

15.  234 

15.  688 

16.  155 

16.  634 

17.  124 

17.  626 

18.  142 

18.  671 

19.  212 

100 

19.  766 

20.  335 

20.  917 

21.514 

22.  125 

22.  751 

94 


TABLE  VI. — Reduction  of  barometer  reading  to  32°. 


Tempera- 
ture. 

Inches. 

24.0 

24.5 

25.0 

25.5 

26.0 

26.5 

27.0 

27.5 

28.0 

+.073 
.070 
.068 
.065 
.063 

.060 

.058 
.055 

28.5 

29.0 

29.5 

30.0 

30.5 

31.0 

+.081 
.078 
.075 
.072 
.070 

.067 
.064 
061 

0 
1 
2 
3 
4 

5 
6 
7 
8 
9 

10 
11 
12 
13 
14 

15 
16 
17 
18 
19 

20 
21 
22 
23 
24 

25 
26 
27 
28 
29 

30 
31 
32 
33 
34 

35 
36 
37 
38 
39 

40 
41 
42 
43 
44 

45 
46 
47 
48 
49 

50 

+  .063 
.060 
.058 
.056 
.054 

.052 
.049 
.047 
.045 
.043 

+.064 
.062 
.060 
.057 
.055 

.052 
.051 
.048 
.046 
.044 

+.065 
.063 
.061 
.058 
.056 

.054 
.052 
.049 
.047 
.045 

+.067 
.064 
.062 
.060 
.057 

.055 
.052 
.050 
.048 
.046 

+  .068 
.065 
.063 
.061 
.058 

.056 
.054 
.051 

+.069  +.070 
.  067  .  t68 
.065   .065 
.  062   .  063 
.060  .061 

.057'  .058 
.  055   .  056 
.  052'  .  053 

+.072 
.Oi.9 
.066 
.064 
.062 

.059 
.057 
.054 

+.075 
.072 
.069 
.066 
.064 

.062 
.059 
056 

+.076 
.073 
.070 
.068 
.065 

.062 
.060 
057 

+.077 
.074 
,072 
.069 
.066 

.063 
.061 
058 

+.078 
.076 
.073 
.070 
.067 

.065 
.062 
059 

+.080 
.077 
.074 
.071 
.068 

.065 
.063 
060 

.049 
.046 

.050 
.047 

.051 
.048 

.052 
.049 

.053 
.050 

.054 
.051 

.054 
052 

.055 
.053 

.050 
.047 
.045 
.042 
.039 

.056 
.054 

.051 
.048 
.045 
.043 
.040 

.057 
.054 

.052 
.049 
.046 
.043 
040 

.058 
.055 

.053 
.050 
.047 
.044 
041 

.041 

.039 
.036 
.034 
.032 

.042 
.039 
.037 
.035 
.033 

.042 
.040 
.038 
.036 
.033 

.043 
.041 
.038 
.036 
.034 

.044 

.042 
.039 
.037 
035 

.045 
.042 
.040 
.038 
.035 

.046 
.043 
.041 
.038 
.036 

.047 
.044 
.041 
.039 
.036 

.047 
.045 
.042 
.040 
.037 

.048 
.046 
.043 
.040 
.038 

.049 
.047 
.044 
.041 
039 

.030 

.028 
.025 
.023 
.021 

.030 
.028 
.026 
.024 
.022 

.031 

.029 
.026 
.024 
.022 

.031 

.029 
.027 
.025 
.022 

.032 

.030 
.027 
.025 
023 

.033  .033 
.030  .031 
.028  .029 
.026   .026 
.  023   .  024 

.034 
.031 
.029 
.026 
024 

.035 

.032 
.030 
.027 
.025 

.035 
.033 
.030 
.027 
025 

.036 
.033 
.  031 

.028 
.025 

.036 
.034 
.031 
.029 
026 

.087 
.035 
.032 
.029 
027 

.038 
.035 
.032 
.030 
027 

.038 
.036 
.033 
.030 
.027 

.019 
.017 
.014 

.019 
.017 
.015 

.020 
.017 
.015 

.020 
.018 
.015 

.020 
.018 
016 

.021 

.018 
.016 

.021 
.019 
.016 

.021 
.019 
.017 

.022 
.019 
.017 

.022 
.020 
.017 

.023 
.  020 
.017 

.023 
.020 
018 

.024 
.021 
.018 

.024 
.021 
.018 
.016 
.013 

.024 
.022 
.019 
.016 
.013 

.012 
.010 

.013 
.010 

.013 
.011 

.013 
.011 

.013 
Oil 

.014 
.011 

.014 
.011 

.014 
.012 

.014 
.012 

.015 
012 

.015 
012 

.015 
012 

.015 
.013 

.008 
.006 
.004 
+.001 
—  001 

.008 
.006 
.004 
+.001 
—  001 

.008 
.006 
.004 
+.001 
—  001 

.008 
.006 
.004 
+.001 
—  001 

.009 

.006 
.004 

+.001 
001 

.009 

.006 
.004 
+.002 
—  001 

.  OO!) 
.006 
.004 
+  .002 
—.001 

.009 

.007 
.004 
+.002 
—.001 

.009 
.007 
.004 
+  .002 
—  001 

.009 
.007 
.004 
+  .002 
—  001 

.010 
.007 
.004 
+  .002 
—.001 

.010 
.007 
.004 
+.002 
—.001 

.010 
.007 
.004 
+  .002 
—.001 

.010 
.007 
.005 
+  .002 
—.001 

.010 
.007 
.005 
+  .002 
—.001 

.003 

003 

.003 

003 

003 

.003 

.003 

.003 

003 

.004 

.004 

.004 

.004 

.004 

.004 

.005 
.007 
.010 
012 

.005 
.008 
.010 
012 

.005 
.008 
.010 
.012 

.005 
.008 
.010 
012 

.006 
.008 
.010 
013 

.006 
.008 
.010 
013 

.006 
.008 
.011 
013 

.006 
.008 
.011 
013 

.006 
.009 
.011 
.014 

.006 
.009 
.011 
014 

.006 
.009 
.012 
.014 

.006 
.009 
.012 
014 

.006 
.009 
.012 
.015 

.007 
.009 
.012 
015 

.007 
.009 
.012 
.015 

.014 
.016 
.018 
.020 
.023 

025 

.014 
.016 
.019 
.021 
.023 

025 

.014 
.017 
.019 
.021 
.024 

026 

.015 
.017 
.019 
.022 
.024 

026 

.015 
.017 
.020 
.022 
.024 

027 

.015 
.018 
.020 
.022 
.025 

-  027 

.016 
.018 
.021 
.023 
.025 

028 

.016 
.018 
.021 
.023 
.026 

028 

.016 
.019 
.021 
.024 
.026 

.029 

.016 
.019 
.022 
.024 
.027 

030 

.017 
.019 
.022 
.025 
.027 

030 

.017 
.020 
.022 
.025 
.028 

030 

.017 
.020 
.023 
.026 
.028 

.031 

.018 
.020 
.023 
.026 
.029 

031 

.018 
.021 
.024 
.026 
.029 

.032 

.027 
.029 
.031 
.033 

.036 
.038 
.040 
.042 
.044 

—.046 

.027 
.030 
.032 
.034 

.037 

.038 
.041 
.043 
.045 

—.047 

.028 
.030 
.033 
.035 

.037 
.039 
.042 
.044 
.046 

—.048 

.029 
.031 
.033 
.035 

.038 
.040 
.042 
.045 
.047 

—.049 

.029 
.032 
.034 
.036 

.039 
.041 
.043 
.046 
.048 

—.060 

.030 
.032 
.035 
.037 

.039 

.042 
.044 
.047 
.049 

—.051 

.0:50 
.033 
.035 
.038 

.040 
.043 
.045 
.047 
.050 

—.052 

.031 
.033 
.036 
.038 

.041 
.043 
.046 
.048 
.051 

—.053 

.031 
.034 
.036 
.039 

.042 
.044 
.047 
.0^9 
.052 

—.054 

.032 
.034 
.037 
.040 

.042 
.045 
.0^8 
.050 
.052 

—.055 

.033 
.035 
.038 
.040 

.043 
.0^6 
.048 
.051 
.054 

-.056 

.033 
.036 
.038 
.041 

.044 
.04C 
.049 
.052 
.054 

-.057 

.034 
.036 
.039 
.042 

.045 
.047 
.050 
.053 
.055 

—.058 

.034 
.037 
.040 
.042 

.045 
.048 
.051 
.053 
.056 

-.059 

.035 
.038 
.040 
.043 

.046 
.049 
.052 
.054 
.057 

-.060 

95 


TABLE  VI — Reduction  of  barometer  reading  to  32° — Continued. 


a 

Inche 

5. 

I5 

24.0 

24.5 

25.0 

25.5 

26.0 

26,5 

27.0 

27.5 

28.0 

28.5 

29.0 

29.5 

30.0 

30.5 

31.0 

50 

—.046 

—.047 

—.048 

—.049 

—  .  050 

—.051 

—.052 

—  .  053 

—.054 

-.055 

—.056 

—.057 

—.058 

—.059 

—.060 

51 

.049 

.050 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

52 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.064 

.065 

.066 

53 

.053 

.054 

.055 

.056 

.057 

.058 

.060 

.061 

.062 

.063 

.064 

.065 

.066 

.067 

.068 

54 

.055 

.056 

.057 

.058 

.060 

.061 

.062 

.063 

.064 

.065 

.067 

.068 

.069 

.070 

.071 

55 

.057 

.058 

.060 

.061 

.062 

.063 

.064 

.065 

.066 

.068 

.06,9 

.070 

.071 

.073 

.074 

56 

.060 

.061 

.062 

.063 

.064 

.065 

.067 

.0(38 

.069 

.070 

.072 

.073 

.074 

.075 

.077 

57 

OfiV! 

.063 

.064 

.065 

.067 

.068 

.069 

.070 

072 

073 

075 

076 

077 

078 

080 

58 

.0(14 

.065 

.066 

.068 

.069 

.070 

.071 

.073 

.074 

.076 

.077 

.078 

.080 

.081 

.082 

59 

.056 

.068 

.069 

.070 

.072 

.073 

.074 

.075 

.077 

.078 

.080 

.'081 

.083 

.084 

.085 

60 

.068 

.070 

.071 

.072 

.074 

.076 

.077 

.078 

.079 

.081 

.082 

.084 

.085 

.086 

.088 

61 

.070 

.072 

.073 

.074 

.076 

.077 

.079 

.080 

.082 

.083 

.085 

.086 

.088 

.089 

.091 

62 

.073 

.074 

.076 

.077 

.079 

.080 

.082 

.083 

.085 

.036 

.088 

.089 

.091 

.092 

.094 

63 

.075 

.076 

.078 

.079 

.081 

.082 

.084 

.085 

.087 

.088 

.090 

.091 

.093 

.095 

.096 

64 

.077 

.078 

.080 

.081 

.083 

.085 

.086 

.088 

.090 

.091 

.093 

.094 

.096 

.097 

.099 

65 

.079 

.080 

.082 

.084 

.086 

.087 

.089 

.090 

.092 

.093 

.095 

.097 

.099 

.100 

.102 

66 

.081 

,083 

.085 

.086 

.088 

.089 

.091 

.093 

.095 

.096 

.098 

.099 

.101 

.103 

.105 

67 

.083 

.085 

.087 

.088 

.090 

.092 

.094 

.095 

.097 

.099 

.101 

.102 

.104 

.106 

.108 

68 

.085 

.087 

.089 

.090 

.093 

.094 

.096 

.098 

.100 

.101 

.103 

.105 

.107 

.108 

.110 

69 

.088 

.089 

.091 

.093 

.095 

.097 

.099 

.100 

.102 

.104 

.106 

.107 

.110 

.111 

.113 

70 

.090 

.092 

.094 

.096 

.097 

.099 

.101 

.103 

.105 

.106 

.109 

.110 

.112 

.114 

.116 

71 

.092 

.094 

.096 

.098 

.100 

.101 

.103 

.105 

.107 

.109 

.111 

.113 

.115 

.116 

.119 

72 

.094 

.096 

.098 

.100 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.116 

.118 

.120 

.122 

73 

.096 

.098 

.100 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.116 

.118 

.120 

.122 

.124 

74 

.098 

.100 

.103 

.105 

•107 

.109 

.111 

.113 

.115 

.117 

.119 

.121 

.123 

.126 

.127 

75 

.101 

.102 

.105 

.106 

.109 

.111 

.113 

.115 

.117 

.119 

.122 

.124 

.126 

.128 

.130 

76 

.103 

.104 

.107 

.109 

.111 

.113 

.116 

.118 

.120 

.122 

.124 

.126 

.128 

.130 

.133 

77 

.105 

.107 

.109 

.111 

.114 

.116 

.118 

.120 

.122 

.124 

.127 

.129 

.131 

.133 

.136 

78 

.107 

.109 

.112 

.113 

.116 

.118 

.120 

.122 

.125 

.127 

.129 

.131 

.134 

.136 

.138 

79 

.109 

.111 

.114 

.116 

.118 

.120 

.123 

.125 

.127 

.129 

.132 

.134 

.137 

.139 

.141 

80 

111 

113 

116 

118 

121 

123 

.125 

127 

.130 

.132 

.135 

.137 

139 

.141 

.144 

81 

.114 

.116 

.118 

.120 

.123 

.125 

.128 

.130 

.132 

.134 

.137 

.139 

.142 

.144 

.147 

82 

.116 

.118 

.121 

.122 

.125 

.128 

.130 

.132 

.135 

.137 

.140 

.142 

.145 

.147 

.149 

83 

.118 

.120 

.123 

.125 

.128 

.130 

.133 

.135 

.138 

.140 

.142 

.145 

.147 

.149 

.152 

84 

.120 

.122 

.125 

.127 

.130 

.132 

.135 

.138 

.140 

.142 

.145 

.147 

.150 

.152 

.155 

85 

.122 

.124 

.127 

.129 

.132 

.134 

.137 

.139 

.143 

.145 

.148 

.150 

.153 

.155 

.158 

86 

.124 

.126 

.128 

.130 

.135 

.137 

.140 

.143 

.145 

.148 

.150 

.153 

.155 

.158 

.161 

87 

.126 

.129 

.132 

.134 

.137 

.139 

.142 

.144 

.148 

.150 

.153 

.155 

.158 

.161 

.163 

88 

.129 

.131 

.134 

.137 

.139 

.142 

.145 

.147 

.150 

.152 

.155 

.158 

.161 

.163 

.166 

89 

.131 

.133 

.136 

.139 

.142 

.144 

.147 

.150 

.153 

.155 

.158 

.161 

.164 

.166 

.169 

90 

.133 

.136 

.138 

.141 

.144 

.147 

.150 

.153 

.155 

.157 

.161 

.164 

.166 

.169 

.172 

91 

.135 

.138 

.141 

.143 

.146 

.149 

.152 

.155 

.158 

.160 

.163 

.166 

.169 

.172 

.175 

92 

.137 

.140 

.143 

.146 

.149 

.152 

.154 

.157 

.160 

.163 

.166 

.169 

.172 

.175 

.177 

93 

.139 

.142 

.145 

.148 

.151 

.154 

.157 

.160 

.163 

.166 

.168 

.171 

.174 

.177 

.180 

94 

.142 

.145 

.147 

.150 

.153 

.156 

.159 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

95 

.141 

.147 

.150 

.153 

.156 

.159 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

.186 

96 

.146 

.149 

.152 

.155 

.158 

.161 

.164 

.167 

.170 

.173 

.176 

.179 

.182 

.185 

.188 

97 

.148 

.151 

.154 

.157 

.160 

.164 

.167 

.170 

.173 

.176 

.179 

.182 

.185 

.188 

.191 

98 

.150 

.153 

.156 

.160 

.163 

.166 

.169 

.172 

.175 

.178 

.181 

.185 

.188 

.191 

.194 

99 

.152 

.155 

.159 

.162 

.165 

.168 

.171 

.175 

.178 

.181 

.184 

.187 

.190 

.194 

.197 

100 

—.154 

—.157 

—.161 

—.164 

—.167 

—.171 

-.174 

-.177 

—.180 

—.184 

-.187 

—.190 

-.193 

—.197 

—.200 

96 


TABLE  VII. — Table  for  reducing  observations  of  the  barometer  to  sea-level,  correction  additive. 


a 

Temperature  of  external  air—  degrees  Fahrenheit. 

•El 
.1 

—20° 

—10° 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

100° 

10 

.013 

.013 

.012 

.012 

.012 

.012 

.011 

.011 

.011 

.011 

.010 

.010 

.010 

20 

.026 

.025 

.025 

.024 

.023 

.023 

023 

022 

.022 

021 

021 

020 

020 

30 

.039 

.038 

.037 

.036 

.035 

.034 

.034 

.033 

.032 

.032 

.031 

.030 

.030 

40 

.052 

.050 

.049 

.048 

.047 

.046 

.045 

.  .044 

.043 

.042 

.041 

.040 

.040 

50 

.065 

.063 

.061 

.060 

.059 

.058 

.056 

.055 

.054 

.053 

.052 

.051 

.050 

60 

.077 

.076 

.074 

.072 

.070 

.069 

.068 

.066 

.065 

.063 

.062 

.061 

.059 

70 

.090 

.088 

.086 

084 

082 

.081 

078 

.077 

076 

074 

072 

071 

069 

80 

.103 

.101 

.098 

.096 

.094 

.092 

.090 

.088 

.086 

.084 

.082 

.081 

.079 

yd 

.116 

.113 

.111 

.108 

105 

104 

101 

.099 

097 

095 

093 

091 

089 

100 

.129 

.126 

.123 

.120 

117 

115 

112 

.110 

108 

105 

.103 

101 

099 

110 

.142 

.139 

.135 

.132 

.129 

.126 

.123 

.121 

.119 

.116 

.113 

.111 

.109 

120 

.155 

.151 

.148 

.144 

.140 

.138 

.134 

.132 

.129 

.126 

.124 

.121 

.119 

130 

.168 

.164 

.160 

.156 

.152 

.149 

.146 

.143 

.140 

.137 

.134 

.131 

.129 

140 

.181 

.176 

.172 

.168 

.164 

.161 

.157 

.154 

.151 

.147 

.144 

.141 

.139 

150 

.194 

.189 

.185 

.180 

.176 

.172 

.168 

.165 

.162 

.158 

.  155 

.152 

.149 

160 

.206 

.201 

.197 

.192 

.187 

.183 

.179. 

.176 

.172 

.168 

.165 

.  162 

.158 

170 

.219 

.214 

.209 

.204 

.199 

.195 

.190 

.187 

.183 

.179 

.175 

.172 

.168 

180 

.232 

.227 

.222 

.216 

.211 

.206 

.202 

.198 

.194 

.189 

.185 

.182 

.178 

190 

.245 

.239 

.234 

.228 

.•222 

.218 

.213 

.209 

.204 

.200 

.196 

.192 

..188 

200 

.258 

'  .252 

.246 

.240 

.234 

.229 

.224 

.220 

.215 

.210 

.206 

.202 

.198 

210 

.271 

.264 

.258 

.252 

246 

240 

.235 

.231 

226 

.221 

.216 

.212 

208 

220 

.284 

.277 

.270 

.264 

.257 

.252 

.246 

.242 

.236 

.231 

.227 

.222 

.218 

230 

.296 

.289 

.283 

.276 

269 

263 

.257 

253 

247 

.242 

.237 

.232 

228 

240 

.309 

.302 

.295 

.288 

.281 

.275 

.269 

.264 

.258 

.252 

.248 

.242 

.238 

250 

.322 

.314 

.307 

.300 

.293 

.286 

.280 

.275 

.269 

.263 

.258 

.253 

.248 

260 

.335 

.327 

.319 

311 

304 

297 

291 

285 

279 

.273 

.268 

.263 

257 

270 

.348 

.339 

.331 

.323 

.316 

.309 

.302 

.296 

.290 

.284 

.278 

.273 

.267 

280 

.3(50 

.352 

.344 

.335 

.328 

.320 

.314 

.307 

.301 

.294 

.288 

.283 

.277 

290 

.373 

.364 

.356 

.347 

.339 

.332 

.325 

.318 

.311 

.305 

.299 

.293 

.287 

300 

.386 

.377 

.368 

.359 

.351 

.343 

.336 

.329 

.322 

.315 

.309 

.303 

.297 

310 

.399 

.389 

.380 

.371 

.363 

.354 

.347 

.340 

.333 

.326 

.319 

.313 

.307 

320 

.412 

.402 

.392 

.383 

.374 

.366 

.358 

.351 

.343 

.336 

.329 

.323 

.317 

330 

.424 

.414 

.404 

.395 

.386 

.377 

.369 

.362 

.  .354 

.347 

.340 

.333 

.326 

340 

.437 

.427 

.416 

.407 

.397 

.389 

.380 

.373 

.365 

.357 

.350 

.343 

.336 

350 

.450 

.439 

.429 

.419 

.409 

.400 

.392 

.384 

.376 

.368 

.360 

.353 

.346 

360 

.463 

.451 

.441 

.430 

.421 

.411 

.403 

.394 

.386 

.378 

.370 

.363 

.356 

370 

.476 

.464 

.453 

.442 

.432 

.423 

.414 

.405 

.397 

.389 

.380 

.373 

.366 

380 

.488 

.476 

.465 

.454 

.444 

.434 

.425 

.416 

.408 

.399 

.391 

.383 

.375 

390 

.501 

.489 

.477 

.466 

.455 

.446 

.436 

.427 

.418 

.410 

.401 

.393 

.385 

400 

.514 

.501 

.489 

.478 

.467 

.457 

.447 

.438 

.429 

.420 

.411 

.403 

.395 

410 

.527 

.513 

.501 

.490 

.479 

.468 

.458 

.449 

.440 

.430 

.421 

.413 

.405 

420 

.539 

.526 

.513 

.502 

.490 

.480 

.469 

.460 

.450 

.441 

.431 

.423 

.415 

430 

.552 

.538 

.525 

.513 

.502 

.491 

.480 

.470 

.461 

.451 

.442 

.433 

.425 

440 

.565 

.551 

.537 

.525 

.513 

.502 

.491 

.481 

.471 

.462 

.452 

.443 

.434 

450 

.578 

.563 

.550 

.537 

.525 

.513 

.503 

.492 

.482 

.472 

.462 

.453 

.444 

460 

.590 

.575 

.562 

.549 

.537 

.525 

.514 

.503 

.493 

.482 

.472 

.463 

.454 

470 

.603 

.588 

.574 

.561 

.548 

.536 

.525 

.514 

.503 

.493 

.482 

.473 

.464 

480 

.616 

.600 

.586 

.572 

.560 

.547 

.536 

.524 

.514 

.503 

.493 

.483 

.474 

490 

.628 

.613 

.598 

.584 

.571 

.559 

.547 

.535 

.524 

.514 

.503 

.493 

.483 

500 

.641 

.625 

.610 

.596 

.583 

.570 

.558 

.646 

.535 

.524 

.513 

.503 

.493 

510 

.654 

.637 

.622 

.608 

.594 

.581 

.569 

.557 

.545 

.534 

.523 

.513 

.503 

520 

.666 

.650 

.634 

.620 

.606 

.593 

.580 

.568 

.556 

.545 

.533 

.523 

.513 

530 

.679 

.662 

.646 

.631 

.617 

.604 

.591 

.578 

.566 

.555 

.544 

.533 

.522 

540 

.691 

.675 

.658 

.643 

.629 

.615 

.602 

.589 

.577 

.565 

.554 

.543 

.532 

550 

.704 

.687 

.670 

.655 

.640 

.626 

.613 

.600 

.587 

.575 

.564 

.553 

.542 

560 

.717 

.699 

.683 

.667 

.652 

.638 

.624 

.611 

.598 

.586 

.574 

.563 

.552 

570 

.729 

.712 

.695 

.679 

.663 

.649 

.635 

.622 

.608 

.596 

.584 

.573 

.562 

580 

.742 

.724 

.707 

.690 

.675 

.660 

.646 

.632 

.619 

.606 

.595 

.583 

.571 

590 

.754 

.737 

.719 

.702 

.686 

.672 

.657 

.643 

.629 

.617 

.605 

.593 

.581 

600 

.767 

.749 

.731 

.714 

.698 

.683 

.668 

.654 

.640 

.627 

.615 

.603 

.591 

610 

.780 

.761 

.743 

.726 

.709 

.694 

.679 

.665 

.650 

.637 

.625 

.613 

.601 

620 

.792 

.774 

.755 

.738 

.721 

.705 

.690 

.675 

.661 

.648 

.635 

.623 

.611 

630 

.805 

.786 

.767 

.749 

.732 

.717 

.701 

.686 

.671 

.658 

.645 

.633 

.620 

640 

.817 

,798 

,779 

,761 

,744 

,728 

,712 

.697 

.682 

.668 

.655 

.643 

.630 

97 


TABLE  VII. — For  reducing  observations,  etc. — Continued. 


_g 

Temperature  of  external  air  —  degrees  Fahrenheit. 

|1 
H 

—20° 

—10° 

0° 

10° 

20°         30° 

40° 

50° 

60° 

70° 

800 

90° 

100° 

i 

650 

.830 

.811 

.791 

.773 

.755 

.739 

.723 

.708 

.692 

.679 

.666 

.653 

.640 

660 

.843 

.823 

.803 

.785 

.767 

.750 

.734 

.718 

.703 

.689 

.676 

.662 

.650 

670 

.855 

.835 

.815 

.797 

.778 

.761 

.745 

.729 

.713 

.699 

.686 

.672 

.660 

680 

.868 

.847 

.827 

.808 

.790 

.773 

.756 

.740 

.724 

.709 

.696 

.682 

.669 

690 

.880 

.860 

.839 

.820 

.801 

.784 

.767 

.750 

.734 

.720 

.706 

.692 

.679 

700 

.893 

.872 

.851 

.832 

.813 

.795 

.778 

.761 

.745 

.730 

.716 

.702 

.689 

710 

.905 

.884 

.863 

.844 

.824 

.806 

.789 

.772 

.755 

.740 

.726 

.712 

.698 

720 

.918 

.896 

.875 

.855 

.836 

.817 

.800 

.782 

.766 

.751 

.736 

.722 

.708 

730 

.930 

.909 

.887 

.867 

.847 

.829 

.811 

.793 

.776 

.761 

.746 

.732 

.718 

740 

.943 

.921 

.899 

.879 

.859 

.840 

.822 

.804 

.787 

.771 

.756 

.742 

.728 

750 

.955 

.933 

.911 

.891 

.870 

.851 

.833 

.815 

.797 

.782 

.767 

.752 

.738 

760 

.968 

.945 

.922 

.902 

.881 

.862 

.843 

.825 

.808 

.792 

.777 

.761 

.747 

770 

.980 

.957 

.934 

.914 

.893 

.873 

.854 

.836 

.818 

.802 

.787 

.771 

.757 

780 

.993 

.970 

.946 

.926 

.904 

.885 

.865 

.847 

.829 

.812 

.797 

.781 

.767 

790 

1.005 

.982 

.958 

.937 

.916 

.896 

.876 

.857 

.839 

.823 

.807 

.791 

.776 

800 

1.018 

.994 

.970 

.949 

.927 

.907 

.887 

.868 

.850 

.833 

.817 

.801 

.786 

810 

1.030 

1.00*6 

.982 

.961 

.938^ 

.918 

.898 

.878 

.860 

.843 

.827 

.811 

.796 

820 

1.043 

1.018 

.994 

.972 

.050 

.929 

.909 

.889 

.871 

.854 

.837 

.821 

.805 

830 

1  055 

1.031 

1.  006 

.984 

.961 

.940 

920 

900 

.881 

864 

847 

831 

.815 

840 

1.068 

1.043 

1.018 

.995 

.973 

.951 

.931 

.911 

.892 

.874 

.857 

.841 

850 

1.080 

1.055 

1.030 

1.007 

.984 

.962 

.942 

.922 

.902 

.885 

.867 

.851 

.835 

860 

1.093 

1.067 

1.041 

1.019 

.995 

.974 

.952 

.932 

.913 

.895 

.877 

860 

.844 

870 

1.105 

1.079 

1.053 

1.030 

1.007 

.985 

.963 

.943 

.923 

.905 

.887 

.870 

.854 

880 

1.118 

1.092 

1.065 

1.042 

1.018 

.996 

.974 

.954 

.934 

.915 

.897 

.880 

.864 

890 

1.130 

1.104 

1.077 

1.053 

1.030 

1.007 

.985 

.964 

944 

926 

907 

890 

.873 

900 

1.143 

1.116 

1.089 

1.065 

1.041 

1.018 

.996 

.975 

.955 

.936 

.917 

.900 

.883 

910 

1.155 

1.128 

1.101 

1.077 

1.052 

1.029 

1.007 

.986 

.965 

.946 

.927 

.910 

.893 

920 

1.168 

1.140 

1.113 

1.088 

1.064 

1.040 

1.018 

.996 

.976 

.956 

.937 

.920 

.902 

930 

1.180 

1.152 

1.  125 

1.100 

1.075 

1.051 

1.029 

1.007 

.986 

.967 

.947 

.929 

.912 

940 

1.193 

1.164 

1.137 

1.111 

1.086 

1.062 

1.040 

1.017 

.997 

.977 

.957 

.939 

.921 

950 

1.205 

1.177 

1.149 

1.123 

1.098 

1.074 

1.051 

1.028 

1.007 

.987 

.967 

.949 

.931 

960 

1.217 

1.189 

1.160 

1.135 

1.  109 

1.085 

1.061 

1.039 

1.017 

.997 

.977 

.959 

.941 

970 

1.230 

1.201 

1.172 

1.146 

1.120 

1.096 

1.072 

1.049 

1.028 

1.007 

.987 

.969 

.950 

980 

1.242 

1.213 

1.184 

1.158 

1.131 

1.107 

1.083 

1.060 

1.038 

1.018 

.997 

.978 

.960 

990 

1.255 

1.225 

1.196 

1.169 

1.143 

1.118 

1.094 

1.070 

1.049 

1.028 

1,007 

.988 

.969 

1,000 

1.267 

1.237 

1.208 

1.181 

1.154 

1.129 

1.105 

1.081 

1.059 

1.038 

1.017 

.998 

.979 

1,010 

1.279 

1.249 

1.220 

1.192 

1.  165 

1.140 

1.116 

1.092 

1.069 

1.048 

1.027 

1.008 

.989 

1,020 

1.292 

1.261 

1.232 

1.204 

1.177 

1.151 

1.127 

1.102 

1.080 

1.058 

1.037 

1.018 

.998 

1,030 

1.304 

1.273 

1.243 

1.215 

1.188 

1.162 

1.137 

1.113 

1.090 

1.069 

1.047 

1.027 

1.008 

1,040 

1.317 

1.285 

1.255 

1.  227 

1.199 

1.173 

1.148 

1.123 

1.101 

1.079 

1.057 

1.037 

1.017 

1,050 

1.329 

1.298 

1.267 

1.238 

1.211 

1.184 

1.159 

1.134 

1.111 

1.089 

1.067 

1.047 

1.027 

1,060 

1.341 

1.310 

1.279 

1.250 

1.222 

1.195 

1.170 

1.145 

1.121 

1.099 

1.077 

1.057 

1.037 

1,070 

1.354 

1/322 

1.291 

1.261 

1.233 

1.206 

1.181 

1.155 

1.  132 

1.109 

1.087 

1.067 

1.046 

1,080 

1.366 

1.334 

1  302 

1  273 

1  244 

1  217 

1  191 

1  166 

1  142 

1.120 

1  097 

1  076 

1  056 

1J090 

1.379 

1.346 

1.314 

1.284 

1.256 

1.228 

1.202 

1.176 

1.153 

1.130 

1.107 

1.086 

1.065 

1,100 

1.391 

1.358 

1.326 

1.  296 

1.267 

1.239 

1.213 

1.187 

1.163 

1.140 

1.117 

1.096 

1.075 

1,110 

1.403 

1.370 

1.338 

1.307 

1.278 

1.250 

1.224 

1.198 

1.173 

1.150 

1.127 

1.106 

1.085 

1,120 

1.416 

1.382 

1.350 

1.319 

1.289 

1.261 

1.235 

1.208 

1.184 

1.160 

1.137 

1.115 

1.094 

1,130 

1.428 

1.  394 

1.361 

1.330 

1.301 

1.272 

1.245 

1.219 

1.194 

1.170 

1.147 

1.125 

1.104 

1,140 

1.440 

1.406 

1.373 

1.  342 

1.312 

1.283 

1.256 

1.229 

1.204 

1.180 

1.157 

1.135 

1.113 

1,150 

1.453 

1.418 

1.385 

1.353 

1.323 

1.294 

1.267 

1.240 

1.215 

1.191 

1.167 

1.145 

1.123 

1,160 

1.465 

1.430 

1.397 

1.365 

1.334 

1.305 

1.278 

1.251 

1.225 

1.201 

1.177 

1.154 

1.133 

1,170 

1.477 

1.442 

1.409 

1.376 

1.345 

1.315 

1.289 

1.261 

1.235 

1.211 

1.187 

1.164 

1.142 

1,180 

1.489 

1.454 

1.420 

1.388 

1.357 

1.  327 

1.299 

1.272 

1.245 

1.221 

1.197 

1.174 

1.152 

1,190 

1.502 

1.466 

1.432 

1.399 

1.368 

1.338 

1.310 

1.282 

1.256 

1.231 

1.207 

1.183 

1.161 

1  200 

1.514 

1.478 

1.444 

1.411 

1.379 

1.349 

1.321 

1.293 

1.266 

1.241 

1.217 

1.193 

1.171 

1,210 

1.526 

1.490 

1.456 

1.422 

1.390 

1.360 

1.332 

1.303 

1.276 

1.251 

1.227 

1.203 

1.180 

1,220 

1.  539 

1.  502 

1.467 

1.434 

1.401 

1.371 

1.342 

1.314 

1.288 

1.261 

1.237 

1.212 

1.190 

1,230 

1.551 

1.514 

1.479 

1.445 

1.413 

1.382 

1.353 

1.324 

1.297 

1.271 

1.247 

1.222 

1.199 

1,240 

1.563 

1.526 

1.491 

1.457 

1.424 

1.393 

1.364 

1.335 

1.307 

1.281 

1.257 

1.232 

1.209 

1,250 

1.  576 

1.  538 

1.502 

1.468 

1.435 

1.404 

1.374 

1.345 

1.317 

1.291 

1.266 

1.242 

1.218 

1,260 

1.588 

1.550 

1.514 

1.479 

1.446 

1.415 

1.385 

1.356 

1.328 

1.302 

1.276 

1.251 

1.  228 

1,270 

1.600 

1.562 

1.526 

1.491 

1.457 

1.426 

1.396 

1.366 

1.338 

1.312 

1.286 

1.261 

1.237 

1,280 

1.612 

1.574 

1.538 

1  502 

1.469 

1.437 

1.407 

1.377 

1.348 

1.322 

1.296 

1.271 

1.247 

1.625 

1.586 

1.549 

1.514 

1,480 

1.448 

1,417 

1.387 

1.359 

1.332 

1.306 

1.280 

1.256 

98 


TABLE  VII. — For  reducing  observations,  etc. — Continued. 


a 

Temperature  of  external  air  —  degrees  Fahrenheit. 

ii 

1 

5 

—20° 

—10° 

0° 

10° 

20°     1     30° 

40° 

50° 

60° 

70° 

80° 

90° 

100° 

H 

1 

1,300 

1.637 

.598 

1.561 

1.525 

1.491 

.459 

1.428 

1.398 

.369 

1.342 

1.316 

.290 

1.266 

1  310 

I.G49 

.610 

1.573 

.536 

1.502 

.470 

1   439 

1.408 

.379 

1.352 

1.  326 

.300 

1.275 

1,320 

1.661 

.622 

1.584 

.548 

1.513 

.481 

1.449 

1.419 

.390 

1.362 

1.336 

.309 

1.285 

1,  330 

1.674 

.634 

1.596 

.559 

1.525 

.492 

1.460 

1.429 

.400 

1.372 

1.346 

.319 

1.294 

1,340 

1.686 

.646 

1.608 

.571 

1.536 

.503 

1.471 

1.440 

.410 

1.382 

1.356 

.329 

1.304 

1,350 

1.698 

.658 

1.620 

.582 

1.547 

1.514 

1.482 

1.450 

.420 

1.393 

1.366 

.339 

1.313 

1,360 

1.710 

.669 

1.  631 

.593 

1.558 

1.524 

1.  492 

1.461 

.431 

1.403 

1.375 

.348 

1.323 

1,370 

1.722 

.681 

1.643 

.605 

1.569 

1.535 

1.503 

1.471 

.441 

1.413 

1.385 

.358 

1.332 

1,380 

1.735 

.693 

1.655 

.616 

1.581 

1.546 

1.514 

1.482 

.451 

1.423 

1.395 

.368 

1.342 

1,390 

1.747 

.705 

1.666 

.628 

1.592 

1.557 

1.524 

1.492 

.462 

1.433 

1.405 

.377 

1.351 

1,400 

1.759 

.717 

1.678 

.639 

1.603 

1.568 

1.535 

1.503 

.472 

1.443 

1.415 

.387 

1.361 

1,410 

1.771 

.729 

1.690 

.650 

1.614 

1.579 

1.546 

1.513 

.482 

1.453 

1.425 

.397 

1.370 

1,420 

1.783 

.741 

1.701 

.662 

1.625 

1.590 

1.556 

1.  524 

.492 

1.463 

1.435 

.406 

.380 

1,430 

1.796 

.753 

1.713 

.673 

1.636 

1.601 

1.567 

1.534 

.503 

1.473 

1.444 

.416 

.389 

1,440 

1.808 

1.765 

1.724 

.685 

1.647 

1.612 

1.577 

1.545 

.513 

1.483 

1.454 

.426 

.399 

,450 

1.  820 

1.777 

1.736 

,696 

1.658 

1.623 

1.588 

1.555 

.523 

1.493 

1.464 

1.436 

.408 

,460 

1.832 

1.788 

1.748 

.707 

1.670 

1.633 

1.599 

1.565 

.533 

1.503 

1.474 

1.445 

.418 

,470 

1.844 

1.800 

1.759 

.719 

1.681 

1.644 

1.609 

1.576 

.543 

1.513 

1.484 

1.455 

.427 

,480 

1.857 

1.812 

1.771 

.730 

1.692 

1.655 

1.620 

1.586 

.554 

1.523 

1.493 

1.465 

1.437 

,490 

1.869 

1.824 

1.782 

1.742 

1.703 

1.666 

1.630 

1.597 

.564 

1.533 

1.503 

1.474 

1.446 

1,500 

1.881 

1.836 

1.794 

1.753 

1.714 

1.677 

1.641 

1.607 

1.574 

1.543 

1.513 

1.484 

1.45C 

ITsTOEX. 


Page. 

Anemometer 51 

Anemometer  correction 52 

Aneroid  barometer 41 

Application  form  for  voluntary  observers 11 

Auroras,  instructions  for  observing 61 

Barometer,  aneroid 41 

changes  of  aneroid , 42 

mercurial .' 30 

cistern  of 36 

correction  of  observations 38 

placing  of 31 

observation  of :    32 

reduction  of  observations  to  sea-level 96 

reduction  of  reading  to  32°  — 94 

reduction  to  sea-level 39 

Beaufort  scale 53 

Bench-mark 59 

Climate,  general  phenomena  of 66 

Clouds,  classification 55 

estimates  of  cloudiness 56 

movement  of 57 

Coronse 60 

Dew  point 27 

tables  for 71 

Directions,  special,  to  observers -..  67 

Earthquakes 64 

Evaporation 57 

E  vaporometer,  Piche 58 

Frost  60 

Hygrometer,  hair 30 

Hypsometer 42 

Instructions  for  use  of  tables 69 

Maximum  thermometer 18 

error  of 18 

setting  of 20 

Marks  on  thermometer,  replacing  of  26 

Mean  temperature 24 

Minimum  thermometer 15 

method  of  uniting  column J6 

placing  thermometer 20 

setting  index 18 

Percolation  gauge 49 

Piche  evaporometer 58 

9377 7  99 


100 


"  ,.-*  o€-  0L^0  t^  Page. 

Psychromet^  ^  Ai".  LC.C..  J"...-i.'.^t!L^.c 28 

whirling  of 29 

wetting  of  muslin 29 

Radiation  thermometer,  solar 25 

Rainfall  measurement 46 

Rain  gauge 43 

instructions  for  use 44 

support  for 46 

Relative  humidity 28 

tables  for 81 

River  gauge 58 

Sand  spouts 61 

Snowfall  measurement 48 

Tables,  instructions  for  use  of. 69 

Thermometers 12 

correction  of 13 

fall  of  freezing-point 14 

maximum 18 

minimum    15 

packing  for  transportation  21 

removal  of  air  from  column 14 

replacing  of  marks 26 

rise  of  freezing-point 13 

shelter  for 22 

solar  radiation 25 

Thermoscope . . . ; 26 

Thunderstorm 61 

Tornadoes  ; 61 

Vapor  of  water  in  air 27 

table  of  correction  to  pressure  for  reading  of  barometer 91 

table  of  grains  of  vapor  in  cubic  foot  of  air 93 

table  of  pressure  in  inches 93 

table  of  weight  of  vapor  in  air 93' 

Weather,  state  of 60 

Wind-vane 49 

Vernier  . .  25 


LIST  OF  TABLES. 

Table     I.  Dew  point  from  wet  and  dry  thermometer  readings 71 

II.  Relative  humidity , 81 

III.  Correction  to  vapor  pressure  for  reading  of  barometer 91 

IV.  Vapor  pressure  in  inches 92 

V.  Grains  of  water  vapor  in  a  cubic  foot  of  air 93 

VI.  Reduction  of  barometer  reading  to  32° 94 

VII.  Reduction  of  observations  of  barometer  to  sea-level 96 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  5O  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


Gaylamount 
Pamphlet 

Binder 
Gaylord  Bros.,  Inc. 

Stockton,  Calif. 
T.  M.  Reg.  U.S.  Pat.  Off. 


417958 
U- 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


